Networking Fundamentals

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The industrial IT revolution
The new and more effective information flowing in today's corporate
systemscan provide many competitive benefits. Shorter delivery times,
faster product development, customer focused production and shorter
turnaround times are some of the key benefits of industrial IT
(Information Technology), not to mention rapid access to information
and the potential for accurate processcontrol.
Industry isdeveloping IT toolsthat require a greater degree of infor-
mation at all stages of the process from purchasing to production to
market. The quality of the information highwaysiscurrently one of the
most important prerequisites for improving industry's competitiveness
and efficiency.
The standard of the highway network
New ideas, solutions and systems for the creation of these IT tools are evolving,
but one negative consequence of this dynamism and great diversity is a long-
standing absence of accepted standards, despite several efforts to remedy this.
Every pioneer hascreated hisown standard and the problem of inadequate stan-
dards only becomes evident when computers, machines and other equipment
have to communicate.
Thisinvolvesstandardsat multiple levels, not merely cablesand connectors, but
such matters as data creation and storage, packaging, addressing and transmis-
sion. As well as the manner in which the medium (e.g. a cable) carries informa-
tion and how it isreceived, unpacked and read by the recipient.
When all such aspects function, then data communication has been realised,
which isthe basisof modern IT development.
10
Data communications involve
more than cables and connectors
GB_1-32_4.0 01-01-11 12.15 Sida 10
11
Industrial data communications
M ost of the standardization within data communications has taken place within
office applications in integrated networks for personal computers, mainframe
computers, printers, servers, modemsetc.
The same focushasnot been given to local data communicationswithin indus-
try. Thismay be due to the fact that the lack of standardization and diversification
iseven greater within industry since computers, lathes, measurement equipment,
scales, robots, transportation systemsand different alarm systemsmust be able to
communicate with each other. Furthermore, greater demandsare made on relia-
bility and immunity to interference.
O ne reason for writing this book is to clarify the concepts used in data com-
munications, to explain how it worksand to provide a practical tool for problem-
solving within local industrial data communications. For further information, con-
tact Westermo or attend one of our seminarsor courseson data communications.
Data communications – more and more important
for improving productivity
Asautomation becomesincreasingly common, the demandsbecome greater on
effective communication between the unitsand systemsused to control process-
esand those that actually carry out production and measurement. Data commu-
nication is the backbone that assures improved efficiency and competitiveness,
regardless of whether the application is manufacturing, construction, transporta-
tion or the medical services.
Data communications
keeps the wheels of
industry in motion.
All products marketed
by Westermo are CE-
marked, which means that
the product complies with EC
requirements in all directives
applying to the standard.
Contact
Westermo for a
declaration of conformity
GB_1-32_4.0 01-01-11 12.15 Sida 11
The purpose of data communicationsisto transfer information between
two or more units. As a rule, it is characters (text or numbers) and/or
instructions(commands) which are transmitted, although it can also be
drawingsand pictures.
The simplest level of computer language is binary characters where
each character iscomposed of seven to eight 1’sor 0’s. M ost comput-
ersoperate at thislevel.
Data communications is basically
a matter of ones and zeros
The computer processesbinary characters, made up of onesand zeros.
Each of the charactersiscalled a bit. By combining several bits, a bina-
ry character set can be constructed. The most common system, ASCII, contains
128 characters, each of which is made up of 7 bits. Each of these characters (or
bit patterns) isknown asa byte. Please note that a kilobyte ismade up of 1 024
ASCI I characters.
All communication iscarried out at thislevel, internally within the computer as
well asexternally with other units. Internal communication within the computer is
simple. However, as soon as the computer has to communicate with external
units, a series of factors must be synchronized and controlled to ensure that the
transmission of data takesplace correctly.
See the ASCI I table on page 21.
12
How does data
communications work?
Bits and bytes
byte
bit
GB_1-32_4.0 01-01-11 12.15 Sida 12
13
One bit at a time, or a whole byte?
There are two waysof transmitting data: by parallel or serial transmission.
Parallel transmission is faster and simpler since the entire character with its
8 bitsistransmitted in a single operation using 8 transmission paths, one for each
bit. All communication within the computer itself takesplace via parallel pathsin
the internal data bus, so that an entire character or several characters can be
simultaneously transmitted.
Parallel transmission via a multi-conductor cable (Centronics-type) can only be
carried out at short distances for practical and economic reasons. Therefore, the
majority of all external data communicationsisachieved through serial transmis-
sion, i.e. the bitsare sent, one at a time, on a single transmission path.
Serial transmission placeshigher demandson the receiver and the transmitter
which hasto keep track of when a character startsand endsand of the inherent
sequence of the bits. The transmitter and receiver must transmit and receive at
the same rate. Thisisknown asthe transmission speed and isexpressed in bit/s
(bitsper second).
In order to tell the receiver where a character startsand ends, the transmitter
sendsout extra bits, a start bit and one or several stop bits.
One character at a time or whole sentences?
There are two methods of serial transmission: asynchronous transmission and
synchronoustransmission. In asynchronous transmission, the transmitter transmits
the charactersone at a time, with their respective start and stop bits. The receiv-
er knowsthat each start bit will be followed by a character which hasto be inter-
preted. The stop bit completing the message re-setsthe receiver. About 90-95%
of serial data transmission isasynchronous.
In synchronous transmissionthe entire message issent in an even flow. The rate
ismaintained by a clock signal on a separate wire or modulated on the data sig-
nal.
The advantage of asynchronous transmission is that it is simple and inexpen-
sive. The disadvantage is that it is inefficient in comparison with synchronous
transmission as it contains as much as 20–25% of message content comprising
parity bits.
Parallel and serial
transmission
Asynchronous and
synchronous transmission
Parallel transmission
Serial transmission
Start bit Stopbit
In asynchronous transmission,
one byte is transmitted at
a time. The byte starts with
a start bit and ends with
a stop bit.
In synchronous transmission,
the whole set of data is
transmitted at once, in a
continuous stream.
GB_1-32_4.0 01-01-11 12.15 Sida 13
TD
RD
SG
TD
RD
SG
TD
RD
SG
2
3
7
TD
RD
SG
2
3
7
TD
RD
SG
TD
RD
SG
TD
RD
SG
2
3
7
TD
RD
SG
2
3
7
Transmitters and receivers
Within the field of data communications, we define hardware as transmitters or
receivers. Two units, e.g. a PC and a robot can both be transmittersand receivers,
although thisisseldom possible at the same time.
When communication only takesplace in one direction, e.g. a computer which
sends an ”on/off” instruction to a motor, this is called simplex transmission. O n
the other hand, if the motor then hasto reply that it isfunctioning and report its
speed, duplex transmission isrequired.
Half-duplextransmission meansthat the communicating unitsmust take turns
in sending out signals, i.e. communication can take place in both directions but
not simultaneously.
Full-duplextransmission istwo-way simultaneoustransmission. O ne example is
a telephone converstation where both partiescan speak at the same time.
The right connection
Two terms which recur in data communications are DTE (Data Terminal
Equipment) and DCE (Data Communication Equipment).
Computers and terminals are usually DTEs, modems and communications
hardware are generally DCEs while other equipment such as multiplexers and
printers can be DTEs or DCEs (refer to the relevant equipment manual). DTEs
transmit and receive data on different pins in the connector than DCEs.
Therefore, to avoid common errors when connecting equipment, it is important
to know the definition of the particular item of equipment.
If you connect a DTE with a DCE, the DTE will transmit data on pin 2 while the
DCE will receive data on pin 2 (in spite of the fact that the signal is called TD,
Transmit Data in both cases). If you connect two DCEs, you have to connect pin
2 and pin 3 in order for the transmitter to be connected to the receiver (for more
detailed information turn to page 19).
Transparent communication
When connecting two or more modems together to create a network the
modemsdo not influence the data. ”What goesin one end comesout the other”
describes why the term transparent communication is used. Transparency also
meansthat all unitswill hear all messages.
Master-Slave configuration and addresses
The vast majority of industrial networksare based on a master slave configuration
where one or several masterssequentially send messagesto the slaveswho in turn
respond. Thissequence iscall polling. Asthe system istransparent a prerequisite
for thisprocedure isthat each slave hasitsown address.
A master sendsa message starting with the specific slave’saddress. The slave
recognizesitsaddressand performsthe commandsincluded in the message. In
many protocolsan acknowledgment isreturned to the master who will proceed
to the next slave.
The format of the addressand the message isall part of the protocol used by
the specific control system. The modemsare not concerned with thisfact aslong
asthe signalsconform to the standard of the communicationsprotocol.
If the slavesare unintelligent (no address) so called addressable modemscan
be used.
A message intended for all slavesiscalled a broadcast message. Thiscan typ-
ically be a message from the master instructing all slaves to perform the same
task. An example would be a number of PLCscontrolling sirens. In case of a gen-
eral alarm all sirensshould sound and thiscould be achieved by sending a broad-
cast message.
14
DTE DCE
DCE DCE
Simplex and duplex
Transparent
Communication
Simplex
Half duplex
Full duplex
GB_1-32_4.0 01-01-11 12.15 Sida 14
15
Transmission speeds
The optimum transmission speed is not the same as the fastest possible speed
since the risk of transmission errorsand interference increaseswith an increase in
the transmission speed. It isthe type of cable and the distance which define the
optimum speed. The aim isalwaysto achieve a highly secure and reliable trans-
mission aswell asimmunity to interference.
In order to send digital data signalsover an ordinary copper wire, the signals
must be transformed. The length of the cable will attenuate and alter the signals.
At high speeds, thiseffect will be critical.
Two terms which are easily confused are those used to describe transmission
speeds: bit/sand baud.
The transmission speed is measured in bit/s (data bits per second). Since
approximately 10 bits are required to transmit one character, it is simple to cal-
culate how many charactersare transmitted per second. At a transmission speed
of 9 600 bit/s, about 960 charactersper second are transmitted.
In order to transform the digital signal into a signal which can be transmitted
on the network, a modem is used. The modem transforms (modulates) the sig-
nal and the baud rateindicates
how many times per second
the signal istransformed. Each
transformation is a ”packet”
which issent along the line to
the receiver’s modem which
unpacks (demodulates) the information into digital signals
again.
Short-haul modemsare transparent and the transmission isnot modulated so
that data isreceived exactly asit wastransmitted. The PTT modem can function
as a short-haul modem or with a built-in buffer to hold the bits before they are
sent. For every transmission more than one bit can be sent so the value for trans-
mission speed-bit/s and the transmission times per second-baud differ. If a
modem transmitsat 2 400 baud and there are four bitsin every transmission, the
transmission rate will be 9 600 bits/s.
Modulation
The term, modem, isan acronym of the term modulation, i.e. signal transforma-
tion, and the term demodulation, which isthe recreation of the original signal. The
data signals must be transformed and adapted so that they can be transported
over different typesof cable. The digital signal levels(1’sand 0’s) must be trans-
formed into readable changesfor the selected cable.
There are three typesof modulation:
Frequency modulation, where different frequencies are used to represent a
1 and a 0.
Phase modulation where the phase of the carrier sine wave isshifted abruptly
to represent the 1’sand the 0’s. Thisisthe most common method used for PTT
modemswhich transmit acrossthe telecommunicationsnetwork.
Amplitude modulationusesthe strength of the signal or amplitude peaksto cre-
ate readable 1’sand 0’s.
Phase/Amplitude modulation is a combination that allows more bits per baud
to be transmitted.
Bit/s and baud
Modulation and
demodulation
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
RD TD DCD RTS CTS DTR RD TC
ACCESS
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
RD TD DCD RTS CTS DTR RD TC
ACCESS
Modulated analogue
electrical signal
Amplitude modulation
Frequency modulation
Phase modulation
GB_1-32_4.0 01-01-11 12.15 Sida 15
Handshaking isa way for data communication equipment to control the
flow of data between connected equipment. It becomes necessary
when there isa part of the system that isslower than the rest.
There are two common formsof handshaking. Hardware handshak-
ing referred to as(RTS/CTS) which usesseparate statuslinesto control
data flow and software handshaking referred to as (Xon/Xoff) which
usesextra charactersin the data flow to achieve the control.
If we have a computer connected to a serial printer and the com-
puter is capable of transmitting its data to the printer faster than the
printer isable to print, it isnormal for the printer to have a small buffer
to store this extra data whilst printing, however under certain circum-
stances this buffer will fill up. This is why software or hardware hand-
shaking is needed to tell the computer that it must stop sending data until the
buffer isempty.
Another example isa computer attached to a modem. The data rate between
computer and modem is sometimes much higher than the telephone line can
support so the modem must use handshake signals to tell the computer to slow
down.
Software handshaking.
With software handshaking the printer for example will send a character to be
computer -Xoff when its buffer is full. When the data in the buffer is processed
then the printer will transmit an Xon Character to the computer. The actual char-
actersused have to be defined in some kind of protocol however ASCI I 17 (Xon)
and ASCI I 19 (Xoff) are commonly used.
16
Handshaking
TD
RD
SG
CTS
RD
TD
SG
DCD
TD
RD
SG
RTS
RD
TD
SG
DTR
Xon/Xoff
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 -
MD-42
R+ R- T+ T-
CHANNEL 4
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 -
MD-42
R+ R- T+ T-
CHANNEL 4
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
R+ R- T+ T- T+ T- R+ R-
GB_1-32_4.0 01-01-11 12.15 Sida 16
Hardware handshaking.
Instead of using extra characters in the data flow, RS-232 provides additional
hardware lines to control communication. The most common lines used are
RTS(Request to Send) and CTS (Clear to Send), typically when a computer wish-
esto communicate with a modem.
1. A computer wishesto transmit data so it raisesRTS (+3V to +15V).
No data istransmitted
2. The connected modem registersthe RTS. When it isready to receive data
it raisesitssignal CTS.
3. The computer waitsuntil it seesthe CTS line go high and then transmits
itsdata.
If at any point CTS isdropped by the modem the computer will stop transmitting.
O ther hardware linesare sometimesused, for instance serial printersoften raise
the DTR line to tell the computer to stop sending data because they have run out
of paper.
Hardware signalsare not alwaysused just for handshaking and can be employed
for a number of purposes within data communications. It is also possible that
when connecting two pieces of equipment together a special combination of
crossoversisrequired to ensure that each piece of equipment seesthe right sig-
nalsat the right timesto ensure reliable data communication.
17
DCE DCE DCE
RTS
1
CTS
DTE DTE DTE
TD
3
2
GB_1-32_4.0 01-01-11 12.15 Sida 17
It isnot enough to agree on the appearance of the signalsand on how
they are to be transformed and transmitted. The next level is
to agree on the appearance of the connectorsand the voltage levelsfor
which they are designed, i.e. the physical and electrical interfaces.
Furthermore, there is a logical interface which defines what a signal
means.
The way in which signals fit together, how the communication is
started, how it is terminated, whose turn it is to send or receive data,
how messages are confirmed etc. are controlled by a protocol. M any
different protocolsexist. For example: Profibus, Comli, Modbus.
The physical interface defines how units should be connected to each
other and definesthe appearance of the connector.
The electrical interface defines the electrical levels and what these mean
(1sor 0s).
The logical interface defineshow the signal should be interpreted.
RS-232-C/V.24
The most common interface for data communications via the serial port of
the computer isthe 9- or 25-pin V.24 standard.
V.24 recommends that the cable should be no longer than 15 metres.
At greater distances, up to several kilometres, short-haul modems are used
to transform the V.24 signal into a signal that islessvulnerable to interference.
V.24 (the ITU-T standard) or RS-232-C (the EIA standard) are two standards
which are similar, in principle, see table. V.24 is the physical standard
while V.28 is the electrical standard. For this reason, the interface is sometimes
described asV.24/V.28.
The interface describes and defines the 25-pin male connector
and the signalsand voltagesfor which they are designed.
Interfaces
18
RS-232/V.24
3 V
-3 V
Start
bit
Data
bits
Parity
bit
Stop
bit
N o t a cce p te d le v e l
GB_1-32_4.0 01-01-11 12.15 Sida 18
19
Signals in V.24/RS-232-C
Pin V.24 RS-232 Signal Signal name Direct.
9/25 Code Code DCE
1 1 0 1 A A G N D P ro te ctiv e G ro u n d –
3 2 1 0 3 B A T D Tra n sm itte d d a ta I
2 3 1 0 4 B B R D R e ce iv e d d a ta O
7 4 1 0 5 C A R T S R e q u e st To S e n d I
8 5 1 0 6 C B C T S C le a r To S e n d O
6 6 1 0 7 C C D S R D a ta S e t R e a d y O
5 7 1 0 2 A B S G S ig n a l G ro u n d –
1 8 1 0 9 C F D C D D a ta C a rrie r D e te cto r O
9 – – can be + 12 V –
10 – – can be – 12 V –
11 126 SCF STF Select Transmit Frequency I
12 122 SCB Secondary DCD O
13 121 SBA Secondary CTS O
14 118 SBA Secondary TD I
1 5 1 1 4 D B T C Tra n sm it C lo ck O
16 119 SBB Secondary RD O
1 7 1 1 5 D D R C R e ce iv e C lo ck O
18 – – – –
19 120 SCA Secondary RTS I
4 2 0 1 0 8 /2 C D D T R D a ta Te rm in a l R e a d y I
21 110 CG SQ D Signal Q uality Detect O
9 2 2 1 2 5 C E R I R in g I n d ica to r O
23 111 CH/CI Data Signal Rate Selector O
2 4 1 1 3 D A E C E x te rn a l C lo ck I
25 133 – RFR Ready For Receiving I
The most common signals used in local communication with modems are print-
ed in bold type. The I /O direction indicatesthe direction from the modem (DCE)
where I isan input signal and O an output signal.
The TD (Transmit Data) signal isan outlet in a DTE and an inlet in a DCE.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1
2
3
4
5
6
7
8
9
DTE to DTE or DCE to DCE DTE to DCE
9 Way
D-sub
9 Way
D-sub
3
2
7
8
6
5
1
4
9
25 Way
D-sub
25 Way
D-sub
1
2
3
4
5
6
7
8
20
22
25 Way
D-sub
25 Way
D-sub
1
2
3
4
5
6
7
8
20
22
9 Way
D-sub
9 Way
D-sub
3
2
7
8
6
5
1
4
9
9 Way
D-sub
9 Way
D-sub
3
2
7
8
6
5
1
4
9
25 Way
D-sub
25 Way
D-sub
1
2
3
4
5
6
7
8
20
22
25 Way
D-sub
25 Way
D-sub
1
2
3
4
5
6
7
8
20
22
9 Way
D-sub
9 Way
D-sub
3
2
7
8
6
5
1
4
9
Cable configuration
The picture below shows how the pin configuration for 9- and 25-pole
connectorsshould be made for all combinationsof DTEsand DCEs.
Male D-subs
GB_1-32_4.0 01-01-11 12.15 Sida 19
Explanation of the most important signals
Protective Ground Connector no. 1 isreserved for protective ground
between the devices.
Signal Ground Signal ground isa signal reference and must alwaysbe
connected to connector 7 (25-pin)/connector 5 (9-pin)
in V.24.
Transmitted Data Thissignal transmitsdata from a DTE to a DCE.
Received Data Thissignal isthe data that a modem or a DCE
transmitsto a DTE.
Request to Send Thissignal isa request to send data from a DTE.
The device waitsfor the CTS answer signal.
Clear to Send The answer signal which tellsthe DTE that it isready to
transmit data.
Data Set Ready The signal from a DCE which indicatesthat the device is
switched on, connected and ready.
Data Terminal Ready The same asDSR, although from a DTE.
Data Carrier Detect The output signal from a DCE which indicatesthat there
isa carrier between the devicesand that the connection
isready for communication.
External Clock Thissignal isused in synchronoustransmission when
it isnecessary to clock data. The signal isthe input in
the DCE.
Transmit Clock Transmitsthe DCE clock in synchronoussystems.
Receive Clock Clock received in the DTE for decoding data.
Ring Indicator O utput signal from a modem indicating that it has
received a ring signal.
20
GND
SG
TD
RD
RTS
CTS
DSR
DTR
DCD
EC
TC
RC
RI
GB_1-32_4.0 01-01-11 12.15 Sida 20
21
ASCII
BINARY b
6
b
5
b
4
b
3
b
2
b
1
b
0
HEX 0 1 2 3 4 5 6 7
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
0
1
1
1
0 0 0 0
0 0 0 1
0 0 1 0
0 0 1 1
0 1 0 0
0 1 0 1
0 1 1 0
0 1 1 1
1 0 0 0
1 0 0 1
1 0 1 0
1 0 1 1
1 1 0 0
1 1 0 1
1 1 1 0
1 1 1 1
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
NUL DLE SP 0
@
P p
SOH
É
`
é
DC
1
! 1 A Q a q
STX DC
2
" 2 B R b r
ETX DC
3
4 D T d t
ENQ NAK % 5 E U e u
ACK SYN & 6 F V f v
BEL ETB ' 7 G W g w
BS CAN ( 8 H X h x
HT EM ) 9 I Y i y
LF SUB * : J Z j z
VT ESC + ; K k
FF FS , < L l
CR GS - = M m
SO RS . > N n
SI US / ? O _ o DEL
$
# 3 C S c s
EOT DC
4
[
Ä
{
ä
\
Ö
|
ö
]
Å
^
Ü
}
å
~
ü
ASCI I is the acronym used for American Standard Code for Information Interchange.
Different varietiesof the ASCI I code exist for different languagesaswell asan Extended
ASCI I where the 8th data bit isused.
GB_1-32_4.0 01-01-11 12.15 Sida 21
TD
RD
RTS
DTR
GND
TD
RD
RTS
DTR
GND
RS-232
DEVICE
A
B
RS-485
DEVICE
RS-485
DEVICE
RS-485
DEVICE
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 - +
MD-42
R+ R- T+ T-
CHANNEL 4
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
R+ R- T+ T- T+ T- R+ R-
TD
RD
RTS
DTR
SG
TD
RD
RTS
DTR
SG
TX A
TX B
RX A
RX B
RX A
RX B
TX A
TX B
RS-232
DEVICE
RS-422
DEVICE
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
RD TD DCD RTS CTS DTR RD TC
ACCESS MA-42
TD
RD
RTS
DTR
SG
TD
RD
RTS
DTR
SG
RS-232
DEVICE
TX A
TX B
RX A
RX B
RS-422
DEVICE
RS-422
DEVICE
RS-422
DEVICE
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 - +
MD-42
R+ R- T+ T-
CHANNEL 4
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
R+ R- T+ T- T+ T- R+ R-
V.11/RS-422
V.11/RS-422 isa standard interface which issuitable for industrial appli-
cations. It was created for the construction of multidrop data buses,
between a main computer and a number of terminals. The interface is
balanced, uses a four-wire format and is relatively immune to interfer-
ence. The interface changesthe polarity of the wire pair depending on
whether a 1 or a 0 isto be transmitted.
RS-422 was originally designed to handle 10 units and can now be
set up with up to 32 units connected. The maximum recommended
distance is 1,200 m (100 kbit/s) or 50 m (10 M bit/s). RS-422 can be
integrated with RS-485, RS-423-A and RS-449 via converters.
RS-422 on four-wire
In the RS-422 four-wire system the master transmiter can always be active/
energized irrespective of the state of the slaves. The standard allowssimultaneous
two-way communications.
22
Interfaces for industry
RS-485
RS-422
Te rm in a tio n
GB_1-32_4.0 01-01-11 12.15 Sida 22
Meter
10 kb/s
10 000
1 000
100
10
1 200
100 kb/s 1 mb/s 10 mb/s
+5 V 0 V
R+
R-
23
RS-485 communication
distance
Failsafe
RS-485
RS-485 isan updated version of RS-422 and isused more and more often nowa-
daysasthe standard interface for different units. It isdesigned for data buseswith
up to 32 unitsand issuitable for multidrop networkswith master/slaves. It isrec-
ommended for distancesof up to 1 200 m.
The great advantage of using this interface is that it can reverse the direction
of communication which allows for half-duplex transmission on two-wire lines.
RS-485 is the transmission method underlying many popular fieldbus standards
for instance, Profibus, Bitbusand Interbus-S.
RS-485 on 2-wire
RS-485 can be used with two-wire in various master/slave systems, where every
slave isaddressable. In a two-wire solution the data direction must be controlled.
This can be achieved with a handshaking signal (RTS/DTR) or by means of the
data flow. Connected units must be capable of what is termed tri-state, i.e. a lis-
tener mode, in which the inactive transmitter entersa high-impedance state thus
not loading the line.
Termination and fail-safe
It is recommended to terminate the line with a circuit of equal impedance to the
characteristic impedance of the line. For RS-422 and RS-485 a 120 Ω resistance is
recommended. The termination should be placed asshown on page 22.
The purpose of the termination isto prevent the reflection of data at the ends
of the cable. If there isno active driver on the network the line can be forced into
a known idle state with a fail-safe circuit. Without thiscircuit it islikely that the line
will pick up noise and falsely trigger the receiversleading to problematic commu-
nications.
RS-232 to RS-422/485 converters – The RTS question
When using an RS-232 to RS-422/485 converter it isimportant to remember that
an RS-485 driver sometimeshasto enter tristate or become a receiver. Normally
the RTS signal from the RS-232 circuit isused to control the state of the convert-
er. To work correctly the RTS signal from the RS-232 device must go high for the
duration that data isbeing transmitted from it and go low to allow the converter
to receive any message back. If this signal is not available then it is necessary to
use convertersthat can control the data direction from received data alone.
Installation
• If a twisted wire pair isused, it should be terminated with a 120-ohm resistor.
• The RS-232 cable should not be longer than 15 metresand keep the RS-422/
485 stubsasshort aspossible.
• RS-422/485 guaranteestransmission distancesup to 1 200 m for data ratesup
to 100 kbit/s. Longer distancescan be allowed at lower speeds.
RS-485
+5 V
0 V
B
A
Tristate
Start
bit
Data
bits
Parity
bit
Stop
bit
GB_1-32_4.0 01-01-11 12.16 Sida 23
Distance and short-haul modems
We mentioned earlier that V.24 does not recommend the use of cables longer
than 15 metres. Consequently, short-haul modems are used for longer connec-
tions. Short-haul modems are sometimes called line-drivers or base-band
modems. These modemstransform the V.24 interface into defined signalswhich
are transmitted over twisted pair or fibre-optic cable for distances up to several
kilometres. The receiving short-haul modem transforms the signals into V.24
again. The modems must be of the same standard and must share a common
interface in order to communicate via the cable.
Current loop (TTY)
O ne problem associated with using ordinary copper wire for long-distance com-
munication isthat transmission isrelatively unreliable and the risk of interference
ishigh. The transmission speed often hasto be reduced to maintain reliability. A
tried and tested method of improving reliability which hasbeen in use for a long
time isto transmit an electrical current over the network.
Current loopisthe oldest known method. V.24 signalsare represeted asa pulse
of electrical current or no electrical current in the wire pair. Current loop issome-
times refered to as TTY. To supply each wire pair with current, the transmitter is
either connected asactive and the receiver aspassive, or vice versa. Current Loop
resultsin more reliable communication but isrelatively vulnerable to interference
since the current loop is not balanced, and there is no accepted standard for
Current Loop.
A more advanced method is the balanced 10 mA current loop described
below.
10 mA balanced current loop (W1)
Westermo hasdeveloped itsown improved interface for itsshort-haul modems. The
aim isto improve the reliability and performance of the modemsand reduce their
vulnerability to interference.
Westermo’smodemstransform the signal to be transmitted into a balanced ±10
mA current loop. The connected units are always electrically isolated from each
other by meansof galvanic isolation. The method involveschanging the direction
of the current in the wire pair depending on whether a high or a low signal isto
be transmitted from the V.24 interface. O n the transmitter side, there are two
amplifierswhich drive ±10 mA into the line and on the receiver side there are two
optocouplerswhich read thissignal.
Current alwaysflowsin the loop, even when no data isbeing transmitted, with
the exception of when you chose to use a hardware handshake line to activate the
transmitter and hence provide a way of sending hardware handshake information
acrossa link.
Thismethod isvery reliable, there isvery little risk of interference and data can
be transmitted at distancesof up to 18 km.
24
Current Loop
T+
T -
T+
T -
R+
R -
R+
R -
R+
R -
R+
R -
T+
T -
T+
T -
20 mA
20 mA
GB_1-32_4.0 01-01-11 12.16 Sida 24
25
Why the 10 mA balanced current loop
is less sensitive to interference
A balanced current loop islesssensitive to interference than a non balanced sys-
tem because when noise is applied to the line both wires are effected the same
way and hence the differential between the two wires that encodes the data is
maintained. See drawing.
1. Data in to the transmitter.
2. Voltageson linesA and B are inverted depending on data hence driving
current either one way or the other through the circuit.
3. Common mode interference on the line.
4. Noise superimposesonto data stream.
5. Data isreceived and decoded unchanged from when it wastransmitted (1)
+
-
+
-
A
B
2
1
3
4
5
TD
RD
Wire A
Wire B
GB_1-32_4.0 01-01-11 12.16 Sida 25
26
Unfortunately, all of our problemsare not solved once we have identi-
fied the right method of transmission and the right interface to use. The
biggest problem of data communications still remains to be tackled –
external interference. Interference can cause data losses, transmission
errors and, worst of all, it can damage equipment. Advances in com-
puter technology have led to the use of smaller circuitsand unitswhich
are operated at lower voltages. Thisisvery favourable in termsof ener-
gy conservation and limiting the heat that isgiven off by the equipment
but it also means that the equipment is more sensitive and vulnerable
to power surges. Surveyshave shown that up to 70% of all interference
during data transmission is due to defective installation or interference
from sources near to the equipment, such as neighbouring machinery
or cables. O nly 20% isdue to defectsin the hardware or software. Therefore, most
of the sources of interference are located in the same room as, or near to the
equipment. The other sourcesof interference are external – often originating from
nature itself – in the form of lightning.
The most significant sort of interference isa type known astransients. These are
short but high pulsesof electrical current in the network. The service life of com-
puter equipment which isexposed to transients, from 1 000 V up to 10 kV, for a
period of a few milliseconds, isbound to be short.
Lightning, machinery and fluorescent lights
We know that very high levelsof current are discharged when objectsare direct-
ly hit by lightning. The discharged electricity can propagate and damage power
and PTT lines and worst of all, cause fires. Even if the line is not directly hit by
lightning, electrical equipment can still be affected because the current propagates
great distances along the power lines. That’s why a bulb flickers even when the
lightning isa long way off.
However, external transients are not only caused by lightning. Bulbs can also
flicker when an industrial plant in the neighbourhood startsup or shutsdown its
machinery. This can also cause transients, or power surges, in the electricity net-
work.
Asa rule, most transientsoriginate right on your own premises. M achinery, equip-
ment and fluorescent lighting can cause power surges.
Switching off a fluorescent light can, for example, result in a power surge, due to
stored charge, of up to 3 000 V. Lightning striking an object near to a power line
could result in a surge of up to 6–10 kV.
The problem
of interference
Transients
GB_1-32_4.0 01-01-11 12.16 Sida 26
27
A normal communications circuit board in a computer is designed for ±12 V.
Therefore, transientsare often the reason why computer equipment breaksdown
without apparent reason or why there istemporary interference in transmission.
Transients are the most common causes of disturbances. O nly in 10% of the
cases is the interference due to failures, i.e. long-term undervoltage or power
surgesand power cuts.
Interference in the network
O ne cause of interference during data transmission, which isalmost ascommon,
isproblemswith earth currents. Thisisa particular problem if the network consists
of units which are electrically connected to different fuse panels. The return cur-
rent can take different routes, either the intended route, via earth to the fuse panel
that the unit isconnected to, or it can travel via the serial port’ssignal ground to
another fuse panel.
Earth currentswhich move through the network can cause interference aswell
ascause damage to the circuitsdriving the line.
A communications network consists of physical lines which are many metres
long. These linesare often laid together with other power and PTT lines. An elec-
tromagnetic field is created around each cable conduct-
ing current and this effects adjacent or crossing cables.
Together, these lines create large aerials which can pick
up different typesof disturbance. Recommendationsexist
for the installation of different typesof cablesin order to
minimize electromagnetic interference.
The simplest way of tackling both transients and the
problem with earth currents is to use modems with gal-
vanic isolationwhich electrically isolate the linesand units
without preventing the transmission of signals. Galvanic
isolation prevents transients, lightning and earth currents
from reaching the equipment.
In the above example, the
earth currents can take the
wrong route, via the com-
puter network’s signal
ground to a fuse panel, and
thereby causing interference.
Earth currents
Zero
Protective ground
Zero
Protective ground
GB_1-32_4.0 01-01-11 12.16 Sida 27
28
In any system, electronic signalsare alwaysprone to inter-
ference. Analogue signalstend to be more prone due to
the fact that all pointson the signal carry information- i.e.
amplitude and frequency. Small disturbance to the signal
will cause the receiving system to interpret the signal dif-
ferently to that of the original transmitted signal and give
an incorrect output.
Digital signals are less prone to interference as there
are only two basic states; high or low. However due to
the interaction of the capacitance, resistance and induc-
tance of the cables used to carry the digital signals and
the effectsof external noise, the information contained in the signalscan be dis-
torted until the signal isunrecognisable.
Industrial signals
M any techniques have been developed to overcome the problems associated
with transmitting data over long distance. For example RS-422 and RS-485 are
designed to operate up to 1 200 m.
Even with bussesspecially designed to be used in long distance data commu-
nications applications, electronic interference will always be a problem and in
extreme casesthe expected transmission distance will not be achieved.
There are many practical stepsthat can be taken to improve the susceptibility
of the signalsto interference.
Isolation
In all data communicationsit isessential to galvanically isolate equipment and net-
works from each other to prevent the propagation of transients and other forms
of interference that can cause transmission errorsor damage equipment.
There are several methodsensuring isolation for example relays, transformers,
isolation amplifiers and optocouplers. Incoming transients can also be removed
using protective components such as varistors, capacitors, RC filters and zener
diodes.
Westermo use optocouplers for isolation in their receivers. O ptocouplers pro-
vide better performance than for example differential amplifiers. Transformerspro-
vide isolation on the power source and varistorsand zener diodesare used to sup-
presstransients.
Interference suppression
+
multidropp

+

multidrop
Fast balanced
communication
GB_1-32_4.0 01-01-11 12.16 Sida 28
29
Shielding
Shielded or double shielded cablescan be used to increase the resistance to exter-
nal interference. Under normal circumstances the cable shield should only be
connected to ground at one end.
In some extreme circumstance where high frequency noise is a problem, the
cable can be connected to ground at both ends. However thismethod introduces
a potentially larger problem if there is a potential difference between the points.
If thisisthe case current will start to flow through the shield of the cable and carry
with it any noise on the ground plain.
As an alternative it is sometimes possible to connect one end of the shield to
ground and the other to ground via a small value, high voltage capacitor
Telephone modems and interference
For very long distance data communication PTT modemsare used. These devices
rely on analogue signalsto carry the digital information over the PTT network . As
stated earlier the analogue signals are prone to the effects of noise. To counter
these effectserror checking algorithmsand filtering are used.
Optical fibre cables
An increasingly common means of reducing the effects of noise in an industrial
environment isto use fibre optic cables.
Data transmission via fibre-optic cable tendsto be insensitive to electrical inter-
ference and provide total isolation between systems, so making it an excellent
media for industrial data communications.
There are limitationsasto the maximum distance that can be achieved. Thisis
due to the amount of available power at the transmitter, the loseswithin the cable,
the type of fibre and the sensitivity of the receiver.
Data communications to
RS-422 for 10 Mbit.
Data communications to
RS-232/V.24
CMW=0
Improved transmission with a
modem and twin-screened
cable, with each screen
grounded at one end.
GB_1-32_4.0 01-01-11 12.16 Sida 29
30
Designing networks
A local network for data communications is usually called a LAN or
Local Area Network. Whether the network iswithin one building or con-
nectsseveral buildings, it isconsidered to be local because it isowned
and operated by the user. The local network can, in turn, be connected
through a leased line or by calling PTT circuits, with a public network,
such as a regional, national and global network, which is sometimes
called a WAN, Wide Area Network or a MAN, Metropolitan Area
Network.
A local network can consist of data communicationsfor office appli-
cations or for industry, hospitals, mining operations or traffic control.
The particular network design which is selected, also known as the
topology, is important since it is a long-term infrastructure which must
handle and transport important data without problems. It must also be possible to
modify or expand the network when necessary.
Serial point-to-point
Point to point data communications, i.e. between two communicating units on
a line, isthe most common application. Thisisthe case in simple applications, such
as computer-printer, as well as in more complex applications, where each user
communicates via his own line for security reasons. The common RS-232 stan-
dard interface isnot recommended for distancesexceeding 15 metres. Therefore,
modemsare used asrepeatersand asprotection against transientsfor communi-
cation at distancesof up to 18 kilometres.
Star network
A network with many point-to-point connectionsiscalled a star network. Each unit
communicates, over itsown line, with the central processing unit at the hub. The
advantage of a star network isthat it ishighly reliable. If one line isdown, the other
linesare not affected. The disadvantage isthat since more cable hasto be used,
thiskind of network ismore costly. Furthermore, all communication must
take place via the central processing unit.
Point to point
Star network
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
GB_1-32_4.0 01-01-11 12.16 Sida 30
31
Ring network
In a ring network, all units are connected in series to each other in a closed cir-
cle. Thismeansthat all communication must passthrough all the other unitsin
the ring in order to reach the receiver. To avoid collisions, an ”empty letter tray”
is circulated around the network. The sender checks that the tray is empty,
inserts an ”address label” and a message. The next unit in the ring checks
whether the message isfor that addressand, if not, passesit on to the next
unit. When the tray reaches its intended destination, the receiver empties it,
insertsa ”receipt” and circulatesthe tray around the network again. The sender
checks the ”receipt” to ensure that the message has been received and then
passes on the empty tray to receive new messages. Token Ring is an example
of a signal-based ring network which isphysically connected like a distributed star
network.
The performance of a ring network ishigh, but it can be more complex to con-
struct and modify than a busnetwork.
Bus or multidrop network
A busnetwork basically consistsof a trunk line to which all unitsare connected as
nodes. All data traffic issent out via the busto the receiver.
A bus network must be governed by rules for how a transmitter checks
whether the line isfree and how it must proceed if the transmission should
collide with other traffic, e.g. through delayed re-transmission.
The busnetwork issimple to install, expand and extend. Examples
of common busnetworksare Ethernet and AppleTalk. O ne of the draw-
backsof thistype of network isthat traffic can be slow if many unitswish to
communicate over the network. However, the busnetwork can be divided into
several sections.
Combined networks
By using different communications products, customized networks can be
designed for specific applicationswhich combine the best featuresof the different
topologies, in terms of performance as well as reliability. O ne such example is a
bus network with a distributed star, which is one way of connecting several star
networks.
It isimportant to take into account the fact that each network needsan effect-
ive system of traffic rulesfor data communications.
For concrete examples of different applications and
network designs, see pages57–102.
Ring network
Bus network
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
T T
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
T
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
PWR RD1 TD1 RD2 TD2 RD3 TD3 RD4 TD4
R D TD D CD RTS CTS DTR R D
ACCESSMX-14
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
LD-01 DC
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 R+ R- T+ T- T+ T- R+ R-
GB_1-32_4.0 01-01-11 12.16 Sida 31
32
Communications layers
Besidestransmitting data (characters, numbers, commands) data communications
equipment must also handle a large quantity of peripheral data which is neces-
sary for communication to take place. For example, such information includesdata
on the transmitter and the receiver (addressing), on what isto be transmitted, on
how it isto be transmitted and on the form into which it isto be transformed and
sent.
For this information to be processed correctly, independently of the type of
communication device used and manufacturer, a reference model, known asthe
OSI (Open Systems Interconnection) model, exists which defines seven different
layersof data communications.
1. The physical layer, which definesthe electrical and mechanical interface.
2. The data link layer, for control and monitoring of the data traffic.
3. The network layer, for handling addressing, paths, performance etc.
4. The transport layer, which handles point-to-point communication, and
also checksthat it isfree from errors.
5. The session layer, which controlsthe data flow and buffering.
6. The presentation layer, which isresponsible for code transformation, for-
matting, conversion and encryption.
7. The application layer, which handles information for the application,
secrecy and identification etc.
The O SI model isnot a standard. Instead it isa reference model for the develop-
ment of different standards.
Industrial fieldbus systems
Ethernet-type buses are most commonly used for office communications and
computer-to-computer communications. This standard is suitable for the type
of transmission which takesplace between several users.
Industrial applicationshave different requirements. Industrial communications
requirements are often less complex while the needs for reliability and perform-
ance are higher. At the same time, communication must be carried out in a harsh-
er environment where there is a high chance of interference. Furthermore, the
communication distancesare long and many different interfacesare involved.
The specification determineswhich network design isselected and which com-
munications protocols are used. The specification also determines which fieldbus
system or systemsare most suitable. Fieldbussystems, such asthe simple ASI and
Can systems, handle simple communications with simple I /O devices. The more
complex Interbus-Sand Profibussystemshandle communicationsbetween one or
several control systemsand between computersand remote-controlled modules.
Furthermore, more or less standardized fieldbus systems exist as well as a num-
ber of control system buseswhich are unique to each supplier.
G ood industrial data communicationssystemscombine different fieldbusstan-
dards(see examplesin Applications).
An industrial system which is commonly used is the multidrop network where
a main computer communicateswith a large number of terminals, units, transmit-
tersor measurement systems.
OSI –
Open Systems
Interconnection
Field Bus Systems
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
7
6
5
4
3
2
1
Communication takes place
via the different OSI layers.
67490 89
ASI
CAN
Profibus
InterbusS
Ethernet
GB_1-32_4.0 01-01-11 12.16 Sida 32
33
Communications products
for industrial networks
The building blocks of a communications network consist of physical cables, the
computer hardware carrying out the communication, computer software as well
asa number of communicationsproductswhich enablesthe data to be transmit-
ted reliable.
Transformsand packsdigital data into signalswhich are defined
for the media which is to transmit the data (4-wire, fibre optic
cablesetc).
Amplifies and restores signals for long-distance transmission.
RS-422 and RS-485 allow connections to be made at a maxi-
mum of 1 200 m with a maximum of 32 loads. By installing a
repeater, you can add a further 1 200 m and 31 load segments
to the network.
Used to save wiring. For example, instead of installing 16 con-
nections with modems and cables, the same function can be
obtained using two multiplexersand one line.
The multiplexer, recreates the 16 channels
and each channel can communicate as if it
was an independent permanent connection
with full-duplex transmission and an optional
transmission speed.
A unit which provides galvanic isolation to isolate connected
devicesfrom each other, often via optical transmission. An iso-
lator does not function as a modem. (With a few exeptions all
of Westermo’sproductsare equipped with galvanic isolation).
Used to enable deviceswith different interfacesto communicate
with each other, e.g. RS-422/485 to RS-232/V.24 or from fibre
optic cablesto RS-422/485 and RS-232/V.24.
A modem with three or four channels, where each channel has
a separate modem function. Used to create multidrop networks.
A router isused to separate different segmentsin a network to
improve performance and reliability.
An intelligent connection between two local networkswith the
same standard but with different typesof cables.
An intelligent connection between a local network and external
networkswith completely different structures.
Multiplexer
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 - +
R+ R- T+ T-
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
R+ R- T+ T- T+ T- R+ R-
CHANNEL 3
PWR
RD
TD
DCD2
DCD3
DCD4
CHANNEL 2 POWER
12-36V DC
1 2 3 4 5 - +
R+ R- T+ T-
1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9
R+ R- T+ T- T+ T- R+
R-
Modem
Repeater
Multiplexer
Isolator
Interface
converter
Line-sharing
device
Router
Bridge
Gateway
GB_33-64_4.0 01-01-11 12.17 Sida 33
34
Data communications over
the telecommunications network
An important complement to local data communications is external
communications. Thisisfor example the possibility of connecting up to
external databasesin order to search for information on markets, to find
out stock-exchange ratesor to accesspublic registersetc.
The number of databaseswhich can be accessed hasincreased enor-
mously and these databases are linked through global networks. For
example, you might connect up to a national database and end up in
an international financial database in New York.
External data communicationscan be justified for many reasons. For
example, the telecommunicationsnetwork isone way of gaining access
to your office and your computer when you are engaged in field work.
Dial-up connections
External communication via the telecommunicationsnetwork meansthat you call
the receiver’smodem which answers. Both modemsthen set up a carrier over the
PSTN line. The carrier is a signal which the modems listen for. When the two
modemsconnect, it meansthat they can hear each other’scarriersand lock into
or ”synchronise” on the signals.
Transmission speedsover the telecommunicationsnetwork have increased, and
nowadays, 2 400–56 000 bit/sare common. It isnot only the modem itself which
determines the speed, but also the PSTN line. The quality of the line is largely
affected by the distance, the number of stations and relays. M ost high-speed
modemscan automatically reduce their speed in order to maintain a high trans-
mission quality.
Within telephone communications, it is very important to comply with the
accepted standards, since the transmitter and receiver have no way of knowing
what devices are being used on either side. The transmission speeds which are
used in certain standardsare shown in the table on the left.
Telephone modem
Standards and speeds
V.21 300 bit/s
V.22 1 200 bit/s
V.22 bis 2 400 bit/s
V.32 9 600 bit/s
V.32 bis 14 400 bit/s
V.34 28 800 bit/s
V.34 bis 33 600 bit/s
V.90 56 000 bit/s
GB_33-64_4.0 01-01-11 12.17 Sida 34
35
The language of the telephone modem
In order to communicate via the telecommunications network, in addition to a
standardized modem, you need a terminal or a computer with communications
software installed which usesthe serial port of the computer.
Instructions are required to control the telephone modem. Hayes M icro-com-
puter Products has developed such a language of instruction which has become
the standard and which is known as the Hayes
®
commands. These are a set of
instructionsto the PSTN modem which can either be manually sent from the com-
puter, via the keyboard, or which are automatically sent from the communications
program when different tasksare carried out.
Error correction and data compression
M ost telephone modems transmit data synchronously between the modems,
even if the communication between the computer and the serial port is asyn-
chronous. In order to monitor the transmission, the data can be divided into
blocks and each block can be assigned a check sum. If there is interference, the
check sum will be incorrect and the receiver will request re-transmission, also
known as an ARQ (Automatic Repeat reQ uest). The most common method of
error correction using the ARQ approach is in accordance with the ITU-T V.42
error correction standard which is supported by both M N P (M icrocom
Networking Protocol) and LAPM (Link AccessProcedure for M odems).
On-line services
The telephone modem can be used to connect to other computers, directly
or indirectly via a network. The internet has rapidly expanded into the largest
world-wide network with millions of users. The TCP/I P protocol used on the
Internet allows electronic mail, discussion groups, world wide web (databases,
information and marketing), file transmission and retrieval, telephony, video con-
ferences, chat etc. However, there are also other networks and services available
via a modem such asM EM O , LotusNotes, CompuServe, etc. The PSTN modem
can be used for distance working by connection to a company'scomputer.
Superhighways
Intensive work isbeing conducted on creating international standardsand on con-
structing " Superhighways" for data communications. Fast digital high-speed net-
works, such as broadband ISDN, can rapidly convey large quantities of informa-
tion containing data, sound and graphics, acrossthe continents. The huge capac-
ity of the cable TV networkscan also offer a new resource for faster data traffic.
However, it is important to remember that the
foundations of such efficient highways must first
be laid locally, through efficient local data com-
munications. With such a vital infrastructure in
place, it will then be possible to open up and
accessnational and global networks.
The fastest communications
route is always in what is
known as direct mode.
Every stage of compression,
error correction and buffer-
ing causes a time delay.
ARQ and MNP
MNP Level 1:
asynchronous protocol,
half duplex
MNP Level 2:
asynchronous protocol, full
duplex. Data divided into
blocks. Actual data transmis-
sion speed somewhat lower
than normal.
MNP Level 3:
synchronous protocol, full
duplex. Data in blocks. 10%
higher speed with error-free
transmissions.
MNP Level 4:
data in blocks, block size
according to line quality.
Smaller blocks than Level 3
which results in a 20% faster
transmission rate, when free
from interference.
MNP Level 5:
as in Level 4, but with data
compression which results in
up to double the speed.
MNP Level 10:
a further development of
MNP 5 which monitors the
line dynamically and guar-
antees error-free transmis-
sion.
CO M P
ERRO R
CO RR
ERRO R
CO RR
CO M P
BUFFER
BUFFER
DSP
DSP
GB_33-64_4.0 01-01-11 12.17 Sida 35
36
Analogue Leased Lines
A leased line isprovided by a telecom company and providesyou with
direct connection between either two sites or multiple sites. Unlike a
dial- up line the leased line is available at all times, but still can go
through exchanges.
There are many configurations of leased lines available. Typically
these will be 2-wire or 4-wire circuits.
O n 2- and 4-wire leased lines, pairsof modemsare used to provide
point to point full duplex communications. These modemswill typical-
ly use the V.22bis, V.32bisor V.34bismodulation standardsto provide
connectionsbetween 2 400 bit/sand 33 600 bit/s. O ne modem will be
set to originate and one modem to answer. O nesa connection isestab-
lished it will remain in place until power isremoved or the line isbro-
ken.
Leased Line V.23
V.23 isan old standard that usesFSK modulation to provide communication cir-
cuitsup to 1 200 baud on 2 or 4 wire circuits.
O ne advantage isthat V.23 can be used in multidrop configurations, either on
dedicated wiresor on specially provided multipoint circuits. V.23 modemsare nor-
mally terminated with a 600 O hm impedance. Thisrestrictsthe number of mul-
tidropped units on a dedicated circuit to about 6 unless these terminators are
removed or line equalisersare employed.
O n V.23 multipoint systemsonly one modem can have an active carrier at one
time so communicationsnormally have to be controlled by an external control sig-
nal such asRTS.
The Westermo solution for V.23
Westermo’s V.23 modem allows speeds up to 1 200 baud. Both the 600 Ω
impedance and complex line impedancesare supported by thismodem.
The carrier, transmit, and input levelsare all adjustable. To avoid problemswith
the line being locked by a faulty unit the modem automatically disconnects from
the line when there hasbeen no activity for a period of time.
A built-in switchable termination makes it possible to connect many more
modemson to one line than the V.23 standard describes.
GB_33-64_4.0 01-01-11 12.17 Sida 36
37
Westermo’s products provide
for secure transmission and
protection against interference
Balanced transmissionfor secure data communications.
Balanced transmission results in error-free transmission over long
distances. Both the Westermo 10 mA balanced current loop and
RS-422/485 use thistechnique.
Reliability that shows
Surveysshow that 70% of the interference during data transmission is
due to internal interference from installations near to the equipment
(transients, earth currents, electromagnetic fields etc.). 10% is due to
lengthy interference in the electricity distribution network (especially in
rural areas) and only 20% isdue to defective software or hardware.
Therefore, a network with Westermo’smodemswhich are equipped
with galvanic isolation providesprotection against the 70% of the inter-
ference which is due to transients, earth currents and electromagnetic
fields. Furthermore, our modems protect your equipment from light-
ning and external transients. Under are the three symbols which are
found on modemsequipped with:
Galvanic isolationwhich through optocouplerselectrically isolatesthe net-
work from your computer equipment. The optocouplershave an isolation
voltage of up to 4 000 V and the optical signal transmission takesplace
via LEDsto transistorswhich detect the signals. Consequently, the optical
isolation of the modems prevents interference and earth currents from
propagating or damaging the equipment.
Transient protectionTransient protection in the network consistsof varis-
torsand on the line, fast two-way Zener-diodeswhich effectively divert
the transientsto protective ground, thereby protecting your equipment
against power surges.
GB_33-64_4.0 01-01-11 12.17 Sida 37
38
ISDN,Integrated Services
Digital Network.
What is ISDN?
ISDN (Integrated ServicesDigital Network) isthe all-digital equivalent
of the conventional telephone network PSTN (Public Switched
Telephone Network), or PO TS (Plain O ld Telephone System).
ISDN technology isstandardized according to recommendationsof
the International TelecommunicationsUnion (ITU),
Signalling
Instead of the phone company sending a ring voltage signal to ring the
bell in your phone (”In-Band signal”), it sendsa digital packet on a sep-
arate channel (”O ut-of-Band signal”). The O ut-of-Band signal does not
disturb established connections, and call setup time isvery fast. The sig-
nalling also indicates who is calling, what type of call it is (data/voice),
and what number wasdialed. Available ISDN equipment isthen capable of mak-
ing intelligent decisionson how to direct the call.
Services
Logically, ISDN consistsof two typesof communicationschannels: bearer service B-
channels, which carry data and servicesat 64 kbit/s; and a single D-channel, which
usually carriessignalling and administrative information which isused to setup and
tear down calls. The transmission speed of the D-channel depends on the type of
ISDN service subscribed to. ISDN servicesavailable today can be divided into two
categories: Basic Rate Interface (BRI) service, which givesthe subscriber accessto two
B-channelsand a 16 kbit/sD-channel; and Primary Rate Interface (PRI) service, which
providesa 64 kbit/sD-channel and 30 B-channelsin Europe and most of Asia, in
North America and Japan the PRI service gives23 B-channels.
When more than one device isconnected through a single ISDN BRI connec-
tion, individual devices are distinguished from one another through the use of
multiple subscriber numbers, (M SN) whereby a different ISDN number is
assigned to each device served by the ISDN subscription.
Up to eight ISDN devicescan be connected on single bus, assignalson the D-
channel automatically take care of contention issues, and route callsand services
to the appropriate ISDN device. Alternatively, a separate sub-address(SUB) value
can be used to differentiate between devices.
GB_33-64_4.0 01-01-11 12.17 Sida 38
39
Speed
The modem was a big breakthrough in computer
communications. It allowed computers to commu-
nicate by converting their digital information into an
analogue signal to travel through the public phone
network. There is an upper limit to the amount of
information that an analogue telephone line can
hold. Currently, it is about 56 kbit/s. Commonly
available modems have a maximum speed of 56
kbit/s., but are limited by the quality of the analogue connection and practically
45–50 kbit/sisreached.
The high throughput offered by ISDN 2 x 64 kbit/s, rapid call setup, lessthan 2 s
and the high level of accuracy inherent to digital transmission, are the main attractions
to ISDN technology.
The two channelscan be bundled to give a virtual 128 kbit channel or used as
two separate channelsenabling simultaneousdata and voice calls.
ISDN Components/Interfaces
I SDN components include terminals, Terminal Adapters (TA), Network-
Termination devices (NT), line-termination equipment (LT), and exchange-termi-
nation equipment (ET). ISDN definesterminalsof two types. Specialized ISDN ter-
minals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals,
such asDTE are referred to asterminal equipment type 2 (TE2). TE1sconnect to
the ISDN network through a 4-wire, twisted-pair digital link. TE2sconnect to the
ISDN network through a TA. The TE2 connectsto the TA via a standard physical-
layer interface such asRS-232/V.24 or RS485/V11.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN net-
work isthe network termination type 1 (NT1) or network termination type 2 (NT2)
device. These are network-termination devicesthat connect the 4-wire subscriber
wiring to the conventional 2-wire local loop. In North America, the NT1 isa cus-
tomer premises equipment (CPE) device. In most other partsof the world, the NT1
is part of the network provided by the carrier. The NT2 is a more complicated
device that typically is found in digital private branch exchanges (PBXs) and that
performsLayer 2 and 3 protocol functionsand concentration services. An NT1/2
device also exists as a single device that combines the functions of a NT1 and a
NT2.
GB_33-64_4.0 01-01-11 12.17 Sida 39
Layer 1 Physical layer
The signalling between the telecom switch and the user isaccording to the U-inter-
face and the signalling in the user building isnormally according to the S-interface.
The U-interface usesframesof 240 bit length. At a rate of 160 kbit/s, each frame is
therefore 1.5 mslong. Each frame consistsof:
Frame structure
U-Frame when 2B1Q coding
ISDN specifiesa number of reference pointsthat define logical interfacesbetween
functional groupings, such as TAs and NT1s. ISDN reference points include the
following:
• R---The reference point between non-ISDN equipment and a TA.
• S---The reference point between user terminalsand the NT2.
• T---The reference point between NT1 and NT2 devices.
• U---The reference point between NT1 devices and line-termination equipment
in the carrier network. The U reference point is relevant only in North America,
where the NT1 function isnot provided by the carrier network.
40
TE1
TA
S/T U V
NT-1
TE1
TE2
R
Network Termination.
Used to convert U to S/T interface
Supplied in Europe by Telco
ISDN equipment
that can connect directly
to ISDN line
S/T interface
Termination point in Europe
ISDN equipment that
can connect NO T
directly to ISDN line
Equipment at phone company switch
Used to connect TE2
devicesto ISDN line
Standard PSTN equipment
hasan R interface
Switch
O /M W1 2 W1 1 W1 W2 S S
D B 1 B 2
8 bits 8 bits 2 bits
240 bits, 1.5 ms
12 words, 216 bits
S = Synchronitation pattern 18 bits
O /M = O peration and M aintance 6 bits
GB_33-64_4.0 01-01-11 12.17 Sida 40
41
Frame Format S interface
ISDN physical-layer (Layer 1) S frame formats differ depending on whether the
frame is outbound (from terminal to network) or inbound (from network to ter-
minal). Both physical-layer interfacesare shown below.
The frames are 48 bits long, of which 36 bits represent data. The bits of an
ISDN physical-layer frame are used asfollows:
Layer 2 – Data Link Layer
The ISDN Data Link Layer is specified by the ITU Q .920 through Q .923. All of
the signalling on the D channel isdefined in the Q .921 spec.
Link Access Protocol – D channel (LAP-D) is the Layer 2 protocol used. This is
almost identical to the X.25 LAP-B protocol.
Here isthe structure of a LAP-D frame:
Flag (1 octet)
Thisisalways7E16 (0111 11102)
Address (2 octets)
SAPI (Service accesspoint identifier), 6-bits(see next side)
C/R (Command/Response) bit indicatesif the frame isa command or a
response
EA0 (AddressExtension) bit indicateswhether thisisthe final octet of the
addressor not
TEI (Terminal Endpoint Identifier) 7-bit device identifier (see next side)
EA1 (AddressExtension) bit, same asEA0
A = Activation bit
B1 = B1 channel
(2 x 8 bits/ frame)
B2 = B2 channel
(2 x 8 bits/ frame)
D = D channel (4 x 1 bit / frame)
E = Echo of previousD bit
F = Framing bit
L = DC balancing
S = S-channel
N = Inverted F from NT to TE
M = M ultiframing bit
B 1 B 2 B 1 B 2 L E
1 1 8 1 1 1 1 1 8 1 1 1 1 1 1 1 8 1 1 1
D A D M D S D E E E L – – –
– – –
F
B 1 B 2 B 1 B 2 D L F L L D L L F L D L L D L L D L
F N
48 bits250µs
NT to TE
TE to NT
Flag Address Control Information CRC Flag
8 7 6 5 4 3 2 1
SAPI (6 bits) C/R EA0
TEI (7 bits) EA1
GB_33-64_4.0 01-01-11 12.17 Sida 41
The figure above gives a
view of usage of the SAPI
field,where SAPI = 0 is
used for switch control and
SAPI = 16 is used for pack-
age routing when X.31,
X.25 over D-channel is used
Control (2 octets)
The frame level control field indicatesthe frame type (Information, Supervisory, or
Unnumbered) and sequence numbers(N[ r] and N[ s] ) asrequired.
Information
Layer 3 protocol information and User data
CRC (2 octets)
Cyclic Redundancy Check isa low-level test for bit errorson the user data.
Flag (1 octet)
Closing flag, always7E16 (0111 11102)
SAPI
The Service Access Point Identifier (SAPI) is a 6-bit field that identifies the point
where Layer 2 providesa service to Layer 3.
TEIs
Terminal Endpoint Identifiers (TEIs) are unique I Ds given to each device (TE) on
an ISDN S/T bus. Thisidentifier can be dynamic; the value may be assigned stat-
ically when the TE isinstalled, or dynamically when activated.
42
SAPI value Related layer 3 or management entity
0 Call control procedures
1–11 Reserved for future standardization
12 Teleaction communication
13–15 Reserved for future standardization
16 Packet communication conforming to X.25 level 3 procedures
17–31 Reserved for future standardization
63 Layer 2 management procedures
All others Not available for Q .921 procedures
TEI Value User Type
0–63 Non-automatic TEI assignment user equipment
64–126 Automatic TEI assignment user equipment
127 Broadcast to all devices
Package data via
D-channel
SAPI-16
Package data via
B-channel
TE ET PH TE
Switch control
SAPI-0
GB_33-64_4.0 01-01-11 12.17 Sida 42
43
These are the fieldsin a Q .931 header:
Protocol Discriminator (1 octet)
Identifiesthe Layer 3 protocol. For a Q .931 header, thisvalue is0816.
Length (1 octet)
Indicatesthe length of the next field, the CRV.
Call Reference Value (CRV) (1 or 2 octets)
Used to uniquely identify each call on the user-network interface. This value is
assigned at the beginning of a call, and thisvalue becomesavailable for another
call when the call iscleared.
Message Type (1 octet)
Identifies the message type (i.e., SETUP, CO N N ECT, etc.). This determines what
additional information isrequired and allowed.
Mandatory and Optional Information Elements (variable length)
Are optionsthat are set depending on the M essage Type.
CAPI
COMMON-ISDN-API (CAPI) is an application programming interface standard
used to accessISDN equipment connected to basic rate interfaces(BRI) and pri-
mary rate interfaces(PRI). By adhering to the standard, applicationscan make use
of well defined mechanism for communications over ISDN lines, without being
forced to adjust to the hardware vendor implementations.
To reflect on the actual situation it can be stated that the international protocol
specification isfinished and almost every telecommunication provider offersBRI /
PRI with protocolsbased on Q .931 / ETSI 300 102. CAPI Version 2.0 wasdevel-
oped to support all Q .931 based protocols.
CAPI isdesigned to be the base of a whole range of new protocol-stacksfor net-
working, telephony and file-transfer and is embodied in European standard ETS
300 838 ”Integrated Service Digital Network (ISDN); Harmonized Programmable
Communication Interface (HPCI) for ISDN”.
Layer 3 – Network Layer
The ISDN Network Layer is specified by the ITU Q .930 through Q .939. Layer 3
is used for the establishment, maintenance, and termination of logical network
connectionsbetween two devices.
Information Field Structure
The Information Field is a variable length field that contains the Q .931 protocol
data.
Information Field
8 7 6 5 4 3 2 1
Protocol Discriminator
0 0 0 0 Length of CRV
Call Reference Value (1 or 2 octets)
0 M essage Type
M andatory & O ptional Information Elements(variable)
GB_33-64_4.0 01-01-11 12.17 Sida 43
44
Radio communications
Wireless data communications via a radio modem provide a means of
maintaining communicationswith
• remote units
• measuring stations
• external buildingsand unmanned installations
• temporary or mobile sites
The purpose may be that of gathering test readings, controlling or reg-
ulating equipment or recording variouskindsof alarms.
Radio communicationstechnology and how to plan, dimension and
cope with noise and interference, differ greatly from local communica-
tionsin a data network.
How it works
Communication equipment is provided using a radio modem that converts the
data signal into radio wavesfor a specific channel with a specific bandwidth. The
data signal may require some form of signal processing or filtering before it can
be transmitted by the radio channel. In addition, the signal is modulated (by a
modem) to a correct carrier frequency and can be transmitted via a radio link to
the receiver. Irrespective of whether the source is analogue or digital, the trans-
mission is nearly always analogue. The receiver equipment decodes and recon-
structsthe original signal.
The available frequency range for radio communications is limited and regu-
lated by an international agreement (ITU).
Radio waves are propagated in the atmosphere in the layer between the iono-
sphere and the surface of the earth. Communication conditionscan vary greatly,
depending on the frequency band, ranging from the longest wavelengths of up
to 1 000 metresin the ELF band to shortest onesof l0 mm in the EHF band.
Radio modemsoperate in the UHF band at around 440 mhz. The UHF band
between 300 and 3 000 mHz also containsradar, radio, TV, N M T mobile teleph-
ony, mobile radio, satellite communications, amateur radio and both G SM and
wirelesstelephones.
Frequency band
ELF 300–3000 Hz
VLF 3–30 kHz
LF 30–300 kHz
MF 300–3000 kHz
HF 3–30 MHz
VHF 30–300 MHz
UHF 300–3000 MHz
SHF 3–30 GHz
EHF 30–300 GHz
GB_33-64_4.0 01-01-11 12.17 Sida 44
45
Attenuation and noise
A propagated radio wave is affected by both the ground and the air layers
through which it passes. In the frequency bandsin which radio modemsoperate,
with wavelengths of around 1 metre, there are many objects such as hills and
buildingsthat can cause a radio shadow (cf. M obile telephony). Thisisin addition
to intermittent interference from other equipment. Such interference caused by
objectsistermed shadow or interference fading, and causessignal attenuation or
distortion.
The signal reaching the receiver is often very weak compared with the trans-
mitted signal but thisin itself doesnot imply any quality deterioration of commu-
nication. What may cause problemsisinterference outside our control, noise that
is added to the signal. This not only occurs in the receiving equipment but also
existsin the form of thermal noise (thermal motion of particles), atmospheric noise
(electrical phenomena such as lightning), cosmic noise (incipient radio-frequency
radiation from the sun or other so-called galactic noise) and locally generated
noise (electrical equipment in the receiver’ssurroundings).
GB_33-64_4.0 01-01-11 12.17 Sida 45
Terminology
When discussing radio communicationsand antenna it isvital to under-
stand a few basic terms and expressions. The first basic formula to
remember relatesfrequency (f) to wavelength (l) by the equation: l [m] =
300 / f [M Hz].
The Radiation pattern isthe three dimensional radiation characteristics
of an antenna in 2 planes, the Electric field (E) and magnetic field (H).
The gain of the antenna isitscapability to force radiation in a specific
direction in space at the expense of other directions. Gain isexpressed in
dB compared to some reference: for example dBi refers to gain com-
pared to an isotropic antenna and dBd to a dipole antenna.
The polarization isdefined asthe plane of antenna’selectric field E and
can be vertical, horizontal, slanted or circular. Typically the antenna’sphysical orien-
tation equals the antenna’s polarization. O rthogonal polarization’s have a cross
polarization lossof 21 dB. In practice all the antennasin one system should use the
same polarization.
The Impedance of an antenna isitsAC-resistance and reactance within the oper-
ating band. Nominal impedance of 50 ohmsisa standard. The bandwidth isthe fre-
quency range where the antenna’scharacteristicslike impedance, gain and radiation
pattern remain within the specifications. The commonly used term attenuation is
mainly related to feedersand radio propagation and isalso expressed in dB.
Antenna circuit components
An antenna is an electromechanical device whose purpose is to radiate as effec-
tively aspossible the power from the feeder in a specific manner.
A power splitter matchesand combinesmultiple loadsor sourcesand equally
splitsthe power between them without disturbing the characteristic impedance of
the system. Splittersare used in antenna arraysto combine multiple antennasor
in RF distribution harnesses. A feed-line isan interconnecting cable between radio
equipment and antenna. Feederstend to be lossy componentsso the type hasto
be carefully selected depending on the required length and operating frequency.
Lightning protectorscan be inserted between the radio equipment and feeder to
help protect the radio against a lightning strike. Typically a lightning protector isa
DC short-circuited quarter wave stub. When interconnecting antenna circuit com-
ponents, impedance match hasto be maintained in order to provide ideal flow of
power without additional lossesdue to reflections. Impedance match iscommon-
ly measured as VSWR (Voltage Standing Wave Ratio) where a VSWR of 1:1 is
ideal and 1:1.5 ismore realistic in practice.
46
Antennas and propagation
in radio communications
GB_33-64_4.0 01-01-11 12.17 Sida 46
Antenna types
Dipolesand dipole arraysare constructed of one or multiple dipole antennasand
power splitterscombining the antennas. These are typically omnidirectional or off-
set pattern antennas.
Yagisand Yagi arraysare constructed of one or multiple yagi
antennasand power splitterscombining the antennas. These are
alwaysdirectional antennas. Cross-polarized yagisare a combina-
tion of two independently fed, orthogonally polarized and physi-
cally quarter wave phased yagi antennas on the same boom.
Cross-polarized yagisare used in applicationswhere polarization
diversity isrequired or in a circular polarization mode when two
yagi antennasare combined with a power splitter.
O mnidirectionalscan be either end fed half wave antennas,
collinear antennas or ground plane antennas. These antennas
radiate in all directionsequally.
Portablesare typically flexible quarter wave antennaswith specific feeding meth-
ods for proper impedance match with small sized portable
radio equipment.
Propagation modes
Radio waves propagate mainly along line of the sight but
there will also be bending, reflection and diffraction occurring.
Typically, radio wavespropagate simultaneously in many dif-
ferent modes and paths. This multi-path propagation causes
some signal instability as a function of time due to the sum-
ming of multiple incoming signals, which have different phas-
es. This also explains why a small physical movement of the
antenna can have influence on indicated signal strength.
The radio horizon is about 15% further than the optical
horizon due to radio wavestendency to bend.
Radio network design
A radio link budget calculation should be performed to see if enough power and
margin isleft at the receiver end of the radio link after propagation. In radio link
calculationseverything isexpressed in dB, plusor minus, and added together.
Radio link budget calculation parametersare distance, frequency, terrain, anten-
na height, transmitter output power, receiver sensitivity, feeder loss, antenna gain
and propagation loss. A radio link budget calculation givesthe same result in both
directions.
Radio network coverage can be improved by using repeaters, which can be
located in suitable positionsand chained to expand the coverage area.
47
Dipole
attenuation diagram
Yagi
attenuation diagram
Example of
Yagi aerial
Example of
Dipole aerial
–3
–12
–15
–18
–21
–24
–9
–6
–27
–30
0
–12
–15
–21
–24
–9
–6
–27
–30
0
–3
–18
GB_33-64_4.0 01-01-11 12.17 Sida 47
48
The Echelon Corporation through itsLonWorkstechnology hasprovid-
ed a platform for developing open control systems offering distributed,
network intelligence.
A LonWorks network is usually a peer-to-peer network where each
device controls its own actions and shares information with its neigh-
boursasneeded to control the entire system.
Normally the nodeswill exchange datarather than commands. In this
approach, application data itemssuch astemperatures, pressures, states,
and other data itemscan be sent to multiple devices, each of which may
have a different application for using that data item. These data items
may be considered as global data variables on the network and are in
LonWorks technology referred to as Network variables. If a device
updatesthe value of a network variable thisnew value will automatically be prop-
agated on the network so other devicesmay be aware of the new value.
Interoperability isthe keyword in the LonWorkstechnology. It isa condition that
ensuresthat multiple devices(from different manufactures) talk the same language
and understand each other on the network.
To achieve interoperability it isnot enough to just be on the same network, have
the same type of transceiver or be able to send and receive network variables. The
nodes must also be able to understand the content of the network variable. For
instance the nodes need to know if the temperature is in degrees, Fahrenheit or
Celsiusor if a flow value isin litre/sec or ml/sec. Hence, a condition for interoperabil-
ity isthat the data itemsare represented in a standardized way. Thishasbeen accom-
plished by the LonM ark Association, an independent organization of LonWorks
developers, system-integrators, and end-users. M embersin the LonM ark Association
work together to define the standards, ensuring interoperability between LonWorks
devicesfrom multiple manufacturers.
In LonWorks technology, interoperability is promoted by the use of Standard
Network Variable Types(SNVT). For a SNVT type network variable, the unit, reso-
lution and range are defined. For example, if a SNVT_speed network variable is
used every LonWorksinteroperable node knowsthat the unit ism/s, the resolution
is0.1 m/sand the range is0 to 6553.5 m/s. Today there are more than 140 dif-
ferent SNVT:s.
LonWorks
®
technology
GB_33-64_4.0 01-01-11 12.17 Sida 48
49
1 2 3
4 5 6
7 8 9
0
Open
10°
20°
30°
40°
50°



• •
5
4
3
2
1
BV
1 4 8 5 4 6 0
SNVT_state
SNVT_switch
SNVT_temp
SNVT_lux
SNVT_time_stamp
SNVT_alarm
The most prevalent transceiver is the FTT-10A free topology transceiver. It com-
municates at a data rate of 78 kbit/s over a twisted pair cable in any topology
including star, bus ring or combinations. The convenience of the free topology
makes it desirable for the interconnection of sensors and controllers in today´ s
control networks. The added benefit isa non-polarized interface eliminatesone of
the biggest problemsin installation today reversing the communicationswires.
O ffering the same flexibility in topology, the LPT-10 link power transceiver can
be powered from the same pair asit communicateson.
There is also a 1 250 kbit/s twisted pair bus transceiver as well as the PLT-22
power line transceiver which hasadvanced signal processing, error correction and
an unique dual carrier frequency feature making it possible to communicate very
effectively in the presence of electric noise, appliance motor noise, dimmers, PCs
and televisions.
LonWorks – a data oriented network
GB_33-64_4.0 01-01-11 12.17 Sida 49
50
The PLT-22 transceiver can be configured for operation in the CEN ELEC A-band
for European utility applicationsor for the CEN ELEC C-band for consumer appli-
cations. There are also transceiversfor fibre optic cable, radio and infrared available.
O ne feature of LonWorkstechnology isthe customer’sfreedom to use multiple
communication media in a control network. Echelon’s routers seamlessly move
LonWorksdata from one medium to another.
Routers interface between a high-speed backbone media and a slower media
providesboth logical and physical network segmentation and protection.
1 2 3
4 5 6
7 8 9
0
Open
10° 20°
30°
40°
50°



• •
5
4
3
2
1
BV
1 4 8 5 4 6 0
Radio
Router
Twisted pair 78 kbit/s
1.25 M bit/stwisted
pair back-bone
Link power twisted pair 78 kb
Power line
GB_33-64_4.0 01-01-11 12.17 Sida 50
51
Applications from simple 78 kbit/s point-point physical layer repeaters/converters
to intelligent 1.25 M bit/sredundant ring topology routersare possible. Fibre optics
providesadvantageslike immunity to electrical interference, reliability and long-dis-
tance communication.
FT-10 Network
FT-10 Network
FT-10
Network
FT-10
Network
FT-10
Network
FT-10
Network
FT-10
Network
FT-10
Network
FT-10 Network FT-10 Network
Redundant fibre optic ring
Fibre Optic Media
Multidrop
Point to point
GB_33-64_4.0 01-01-11 12.17 Sida 51
52
Selecting the right cable
The physical cable is often the weak link in data communications. The
cable isthe medium which handlesthe analogue signal which ishighly
vulnerable to interference. Through itsdesign, installation and length, it
isthe cable and the electrical environment in itsvicinity which determine
the speed and quality of the transmissions.
Twisted pair cable
Twisted pair cable isthe simplest, least expensive and most commonly
used type of cable and ismost often found in the form of a 4-wire twist-
ed pair cable. It consistsof an ordinary copper wire in a protective plas-
tic covering, with or without a protective metal shielding. Such cables
are manufactured by different manufacturers and different types exist
with different levelsof performance. Thisshould be taken into account
when considering the requirementsof the particular installation. Different layersof
insulation exist which are suitable for different environments. Three important
parameters affect the quality of data transmission: resistance, capacitance and
attenuation.
Resistance The electrical resistance of the cable. Resistance is measured in
Ω/km and variesbetween the material of the wire and the surface
area. The resistance isprovided in the technical data for the partic-
ular cable. The diameter of cableswith solid conductorsshould not
be less than Ø 0.5 mm and, for multi-conductor cables, the rec-
ommended area is0.2 mm
2
. At low transmission speedsthe resist-
ance isthe limiting factor.
Capacitance Since the conductorsin the cable are isolated from each other, they
will generate a capacitive connection between each other. Similarly,
the twisted pair, the material of the conductors and any shielding
will also have an effect. The capacitance causes the signals to be
attenuated differently at different frequenciesand the value of 800
Hz isoften given. The capacitance ismeasured in pF/m and a good
rule of thumb when selecting an adequate computer cable isabout
50–70 pF/m. At high transmission speeds the capacitance is the
limiting factor.
Attenuation The cable’s total attenuation of the signal from the transmitter to
the receiver. The attenuation isgiven in dB/km and increases, the
higher the frequency. An decrease by 3 dB meansthat the power
isreduced by half.
Attentuation (examples)
150 kHz 8 dB/km
1 MHz 20 dB/km
4 MHz 40 dB/km
10 MHz 65 dB/km
16 MHz 82 dB/km
25 MHz 105 dB/km
GB_33-64_4.0 01-01-11 12.17 Sida 52
53
Coaxial cables
A coaxial cable consistsof a single copper conductor surrounded by a grounded
shielding. In order to keep the gap between the two constant, it is filled with an
insulating plastic layer of dielectric material. The shielding is used for protection
and for return signals. The coaxial cable hashigh electrical propertiesand issuit-
able for high-speed communication.
From the start, only coaxial cableswere used for Ethernet. There are two types
of coaxial cable: a thick cable (10Base5) and a thin cable (10Base2). Nowadays, a
special 4-wire cable (10BaseT) isincreasingly being used for Ethernet.
The advantage of the coaxial cable over the 4-wire cable isthat it can be used
for broadband transmission, i.e. several channels can be transmitted simultane-
ously (aswith cable TV).
Fibre optic cables
Instead of conducting electric signals, asin the case of the copper cable, the fibre
optic cable conductslight signals. A fibre optic cable can have a light-conducting
core of glass or plastic. The core is surrounded by a thicker layer, called the
cladding, which rendersthe surface area of the core totally refractive, and around
this, there isa protective layer which actsasa buffer for the sensitive core. A dis-
tinction is made between single mode and multi mode fibres. Single mode fibre
hasa very thin core which isused together with a laser for long range high speed
communication. The multi mode fibre issomewhat coarser which allowsfor more
refraction which leadsto lower data ratesand a shorter range. M ulti mode glass
fibre ismost commonly used in local data communicationsapplications.
The greatest advantage of the fibre optic cable isthat it isimmune to electrical
and magnetic interference. Consequently, it is highly suitable for harsh industrial
environments. It guaranteessecure transmission and hasa very high transmission
capacity. Fibre optic cables can be used in certain vulnerable network segments
and can be combined, via a modem, with a 4-wire cable in a network.
A modem for fibre optical transmission transforms the electrical current into
light signalswhich are transmitted over the cable with the help of LEDs. The sig-
nalsare received by a photodiode which re-createsthe electrical signals.
The signals can be modulated for different carriers (frequencies) which create
transmission channels in both directions. Fibre optic cables can also be used for
broadband transmission, where several channels(bandsof frequency) are handled
in parallel and where it is also possible to mix data transmission channels with
channelsfor telephone, graphics, TV and sound.
Fibre optic cable and cable installation are still a bit more expensive than cop-
per but have some major advantages, and the market isgrowing.
Single mode
Multi mode
Core of glass fibres
Copper conductor
Shielding
Dielectric material
Cladding
GB_33-64_4.0 01-01-11 12.17 Sida 53
54
Distance and design
n.b. All speedsand distanceshave
been calculated based on cables
with standard valuesfor resistance
and impedance.
In field installationsthe transmission
distance may vary depending on
cable quality and local conditions.
It isnot alwayseasy to construct bridgesfor data communications. Not
only must different pointsbe connected by a communicationsmedium,
the medium must also be designed to handle current and future traffic
loads. It must also be able to effectively handle certain transmission
speeds, it should not require maintenance and it must be able to with-
stand environmental impact.
Since this is a question of determining the right design for the spe-
cific conditionsof the particular application, it isimpossible to formulate
a general design which can be applied to all areas. The best approach
isto discussdifferent alternativeswith one or several expertsin order to
arrive at an optimum solution.
Distance and speed
Westermo’s modems have been tested using different types of cable with differ-
ent transmission speedsand distances, so that general comparisonscan be made.
The diagram below showsthe transmission speed in bit/s, in relation to the dis-
tance for different cablesand interfaces.
Leased Telephone Line
10mA balanced current
loop (W1)
Fibre optic
RS-422/485
20mA current loop
RS-232/V.24
k
b
i
t
/
s
M
b
i
t
/
s
b
i
t
/
s
5 0 1 0 0 5 0 0 1 2 4 6 8 1 0 1 2 1 4 1 6 1 8 2 0 2 2 2 4
4 .0
5 0 0
1 8 7.5
1 1 5 .2
1 0 0
3 8 .4
1 9 .2
1 4 .4
9 .6
4 .8
2 .4
1 .2
6 0 0
T D -3 2
T D -2 2
T D -2 3
K ilo m e tre s M e tre s
S in g le m o d e
M u lti m o d e
8 2 0 n m
M u lti m o d e
1 3 0 0 n m
GB_33-64_4.0 01-01-11 12.17 Sida 54
55
Calculating the resistance
If you do not know the resistance of a particular cable, you can use this formula
to calculate it:
Q = R x
where Q = the resistivity of the material used. A copper cable would have a resis-
tivity of 0.017µΩm, or 0.017 x 10
-6
. R = resistivity in the cable, A = cable area and
l = length.
Thisformula iseasy to use for solid conductors. For multi-conductor cables, the
area of a conductor ismultiplied by the number of conductors.
Area = radiusx radiusx π
Two symbols for capacitance
Two different symbols are used for capacitance: nF/km or pF/m. These are two
variationsof the same unit of measurement. nF standsfor nano farad which is10
-
9
farad. pF standsfor pico farad which is10
-12
farad.
Cable codes
The Swedish standard of cable coding isprovided in SEN 241701 and a joint inter-
national standard hasbeen formulated in CEBELEC.
The cable ismarked with two to five letterswhich stand for:
A
l
Conductor
Insulation Covering Properties
Other
1 st le tte r
A copper, sin-
gle-wire
B copper, multi-
conductor
K coaxial tube
M copper,
multi-strand
R copper, extra
multi-strand
S copper, fine-
gauge
T copper, extra
fine-gauge
2 n d le tte r
D rubber, outer
rubber tube
E ethylene-pro-
pylene rubber
F fluorocarbon
rubber
H silicone rub-
ber
I polyurethane
K PVC
L polythene
plas-tic
M polypropy-
lene plastic
N polyamide
T polytetrafluo-
ro-ethylene
(PTFE)
U cellular poly-
thene plastic
V rubber, with-
out rubber cov-
ering
2 n d a n d 3 rd
le tte rs
O oil- and
weather-resist-
ant rubber
(chlo-roprene
rubber)
S chlorosulpho-
nated polyethy-
lene
3 rd le tte r
C concentric
copper conduc-
tor
H heat-resistant
braided fabric
I polyurethane
J armoured with
steel band
K PVC sheath
with round
cross-section
L polythene
plastic
N polyamide
T galvanized
steel wire
armour
U without
cover-ing
V rubber
Y covered by a
single insulation
and sheath
3 rd a n d 4 th
le tte rs
A aluminium
band shielding
F metal-
screened cable
P galvanized
steel wire
armour
4 th le tte r
B connection or
cable for vehi-
cles
D sheath with
embedded rein-
forcement and
loose conduc-
tors
H insulated con-
ductors, cabling
around rein-
force-ment
K PVC sheath
L PE sheath
N PA sheath
O oil- and
weather-resist-
ant rubber
sheath (chloro-
prene rubber)
T heavy con-
nec-tion line
V cable to be
laid in water
5 th le tte r
H separately
shielded
conductors
K PVC sheath
L PE sheath
N PA sheath
P separately
sheathed pairs
4 th a n d 5 th
le tte rs
C cable with
reinforcement
embedded in
sheath
E reinforced
design
J cable which
may be laid in
the ground or
steel band
armour with
metallic coating
R control or
signal cable
X PSTN line
Y weather-
resistant PSTN
line
E D C K P
Colour codes
DIN 47100 for LiYY
and LiYCY data cables.
Conductor no. and colour:
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
GB_33-64_4.0 01-01-11 12.17 Sida 55
56
Fibre optic cable design
M any designs of glass fibre cables are available. The most common multi mode
fibres are 50/125 and 62.5/125 (Ø µm core/cladding). For long distances single
mode fibreslike 8/125 and 9/125 are becoming very common.
Plastic fibre and PCS cable are simpler formsof fibre optic cable which are eas-
ier to install and join. They are suitable for short distances and low transmission
speeds.
Besides the diameter, two factors affect the attenuation of the signal in the
cable, namely, the length of the cable and the number of jointsor connectionsin
the network. By calculating the difference between the transmitter’soutput power
and the receiver’ssensitivity, you can determine the power budget for the optical
link:
For example:
When performing a loss budget calculation the maximum budget of a system can
be calculated by subtracting the receiver sensitivity figure from the transmitter
power. Then by subtracting from this figure the total of connection losses in the sys-
tem the maximum range can be calculated. Losses in fibre occur at joints between
fibres. No account should be taken of the connections to the modems as the trans-
mit power and receive sensitivity figure already allow for these connections.
If for example we have a system with a transmit power of –12dBm and a receive
sensitivity of –28dBm then the maximum system budget is 16dB. If in this system
there are 2 patch panels and 2 fusion splices then the overall budget is reduced to
16dB – (2x0.4dB)-(2*0.1dB)=15dB
If the system is using 62.5/125 fibre then the maximum transmission distance would
be
15db / 3.5dB/km = 4.29 km
Fibre optic termination
There are a number of waysto fix a connector onto the end of a fibre optic cable
in the field. The fibre can either be direct terminated or a factory terminated pigtail
can be spliced on to it.
Direct termination requires an epoxy inside the connector to harden and lock
the glassfibre into itsprecise location. The connector isthen crimped to the outer
coating of the fibre to give strength. Finally the excessfibre iscleaved flush to the
connector surface and polished using progressively finer gradesof abrasive papers.
The processof splicing pigtailsallowsthe termination processto be performed
in factory conditionsprior to installation. Thisleadsto splice lossesin the final instal-
lation so when the lowest lossesare required direct termination in the field isthe
best option.
Attenuation, distance
Fibre Attenuation
62.5/125 3.5 dB/km
PCS 6.0 dB/km
9/125 0.5 dB/km
Attenuation, joints
Type Attenuation
Fusion splice 0.1 dB
Mechanical
splice 0.2 dB
Connectors 0.2-0.4dB
Connectors for
fibre optic cables
SC connector
ST connector
SMA connector
GB_33-64_4.0 01-01-11 12.17 Sida 56

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