Asynchronous Transmission

Asynchronous Transmission Overview
Introduction
The ASYNCHRONOUS (ASYNC) format for data transmission is a procedure or protocol in
which each information CHARACTER or BYTE is individually synchronized or FRAMED by
the use of Start and Stop Elements, also referred to as START BITS and STOP BITS.
The Asynchronous Transmission Format is also known as START-STOP mode or
CHARACTER mode. Each character or byte is framed as a separate and independent unit of
DATA that may be transmitted and received at irregular and independent time intervals. The
characters or bytes may also be transmitted as a contiguous stream or series of characters.
The character or byte may contain the number of bits required to allow translation of the BIT
PATTERN into a group of symbols used to represent:
 LETTERS (alpha characters)
 NUMBERS (numerical values)
 PUNCTUATION MARKS
 CONTROL ELEMENTS
Elements of an Asynchronous Data Communication
Network
TERMINAL MODEM MODEM TERMINAL
_____ _____ COMMUNICATIONS _____ _____
| | | | LINK | | | |
| DTE |-<=>-| DCE |__/\ /\ /\ /\__| DCE |-<=>-| DTE |
| | ^ | | \/ \/ \/ | | ^ | |
|_____| | |_____| |_____| | |_____|
^ | `--- NO CLOCK ---' | ^
| | | |
| `------------- INTERFACE ------------' |
| |
`---------------- INTERNAL CLOCK ---------------'
NOTE: If clocking is provided by the modems (DCE), refer to the the Isochronous Transmission
Overview.
The TERMINALS or DTE devices normally communicate with other terminals or DTE devices
across a communications NETWORK via some form of MODEMS (MOdulator-DEModulators)
that are connected through a communications LINK.
The terminals are connected to the modems through an INTERFACE. There are many different
types of interfaces in use due to the differences in the characteristics of the DTE terminals and
the communications links being used, and the performance requirements.
The terminals or DTE devices operating in the Asynchronous Mode will normally provide the
INTERNAL TIMING or CLOCKS required to strobe the data out of and into the modems or
DCE Devices.
The modems or DCE devices operating ASYNC normally do not provide any data timing or
clocks.
NOTE: If the modems (DCE) provide the data timing or clock strobes to the terminals (DTE)
through the interface, the operation is using Isochronous Transmission format and procedures.
Refer to the Isochronous Transmission Overview.
The normal IDLE condition of the communications link is referred to as MARK, which indicates
that there is continuity through the link and that energy is present. The transition from MARK to
the SPACE condition indicates that an event is occurring, either a character is being received or
the communications link has been interrupted.
The transition to a SPACE condition for a defined time period (BIT TIME) will normally
indicate the "start" of a character and is referred to as the START BIT. After the last bit of the
data character, the communications link should "stop" the data and be returned to the MARK
condition for one or more bit times or STOP BITS.
Character Format
Most communications equipment will require a specific number of BITS to be in each data
character or byte, depending upon the equipment, the protocol, and the type of information that is
to be transmitted. Each bit may be set to a BINARY VALUE of either 1 or 0.
A group of 4 bits is referred to as a DIGIT. This group of 4 bits provides 16 different patterns
that are referred to as HEXADECIMAL NOTATION. The basic hexadecimal notation allows a
single 4-bit digit or symbol to represent 16 different values: 0 through 15.
The relative position of each bit will determine the value that is assigned to the specific bit
which, in turn, will determine the value of the digit. The combination of two 4-bit digits will
form an 8-bit BIT CHARACTER or BYTE that may be processed and displayed as a symbol.
The table on the next page shows the binary value, decimal value, and symbol for each of the 16
hexadecimal digits.
SYMBOL DECIMAL 8 4 2 1 <-- BINARY VALUE
--:--------:------:---:---:---:-----------------
0 0 0 0 0 0
1 1 0 0 0 1
2 2 0 0 1 0
3 3 0 0 1 1
4 4 0 1 0 0
5 5 0 1 0 1 HEXADECIMAL
6 6 0 1 1 0
7 7 0 1 1 1 NOTATION
8 8 1 0 0 0
9 9 1 0 0 1 FORMAT
A 10 1 0 1 0
B 11 1 0 1 1
C 12 1 1 0 0
D 13 1 1 0 1
E 14 1 1 1 0
F 15 1 1 1 1
Most of the existing equipment now uses a character or byte that contains 8 bits, consisting of
two 4-bit digits that represent a specific symbol, letter, number, or function depending upon the
type of translation (CODESET) used. The digits are referred to as belonging to a COLUMN
(COL) and a ROW (ROW) as presented on many code translation charts.
The number of data bits per character may be five (5), six (6), seven (7), or eight (8). The most
commonly used 8-bit format uses 7 DATA BITS with the 8th bit, referred to as the PARITY
BIT, reserved for error-checking purposes. This type of error checking is called PARITY
CHECKING.
One of the most commonly used TRANSLATION or CODESETS used with this format is the 7
data bit ASCII plus 1 parity bit. Other codesets that may use character parity are BAUDOT and
EBCD.
Character Parity
The PARITY BIT is used to establish the number of bits that are set to the value of 1. Some
common CHARACTER PARITY algorithms are identified as:
EVEN
The EVEN parity algorithm specifies that the character must
have an EVEN number of 1 bits. Referred to as "7 EVEN" (7-
E).
ODD
The ODD parity algorithm specifies that the character must have
an ODD number of 1 bits. Referred to as "7 ODD" (7-O).
SPACE
The SPACE parity algorithm specifies that the PARITY BIT of
the character must have a value of 0, the "space" condition. This
is referred to as "7 SPACE" (7-S).
MARK
The MARK parity algorithm specifies that the PARITY BIT of
the character must have a value of 1, the "mark" condition. This
is referred to as "7 MARK" (7-M).
NONE
Some procedures will use all 8 bits for data or may not provide
error checking. Therefore, NONE of the bits are used for parity
and all 8 bits are considered to be data. This technique is
referred to as "EIGHT NONE" (8-N).
Character parity is also referred to as VERTICAL PARITY. The vertical parity error-checking
algorithms will report an error if the CHARACTER does not contain the correct number of 1 bits
in the correct positions. This is displayed by a BAR through the parity-flawed character.
The Asynchronous Format data character will normally contain 8 DATA BITS plus 1 START
BIT and include at least 1 STOP BIT, for a total of 10 bits.
If 2 STOP BITS are used, then each character will contain 11 bits.
Transmission Speed and Timing
TRANSMISSION SPEEDS are expressed in the number of bits that are transmitted per unit of
time, usually in BITS PER SECOND (bps). The FLOW of the number of CHARACTERS PER
SECOND is dependent upon the number of bits required to form one character.
The accuracy of the BIT TIMES is very critical: all bit times must remain within a narrow range
to ensure accurate and error- free communications. The allowable bit-time variation is normally
less than 3%.
The SENDING data terminal equipment (DTE) must generate the bit timing using a very precise
INTERNAL Clock, usually crystal controlled, so that all bit times are of equal duration and
operate at a constant repetition rate.
The RECEIVING DTE must use the same defined normal bit-timing clock speed as the
SENDING DTE; these two clocks must be operating at the same or matched speed. The
receiving DTE will sense the beginning of the START BIT and then sample each succeeding bit
near the OPTIMUM CENTER of the bit time.
The RECEIVING DTE sample timing or STROBE is generated by using another internal clock,
usually operating at speeds 16 or 32 times as fast as the normal bit-timing clock.
Some common speeds with the corresponding bit times and character rates are shown in the table
on the next page.
SPEED BIT TIME CHARACTER RATE (cps)
(bps) 10 BIT 11 BIT 8 BIT (SYNC)
------ ---------- ------ ------- -------------
110 9.09 mSEC 11 10 14
150 6.666 mSEC 15 13.6 19
300 3.333 mSEC 30 27.3 37.5
600 1.666 mSEC 60 54.5 75
1200 833 uSEC 120 109.1 150
2400 416 uSEC 240 218.2 300
3600 278 uSEC 360 327.3 450
4800 208 uSEC 480 436.4 600
9600 104 uSEC 960 872.7 1200
19200 52 uSEC 1920 1745.5 2400
48000 21 uSEC 4800 4363.6 6000
56000 18 uSEC 5600 5090.1 7000
64000 16 uSEC 6400 5818.2 8000

mSEC = MILLISECONDS uSEC = MICROSECONDS
Bit Sense
The MARK condition is normally established by a NEGATIVE voltage on the interface. The
SPACE condition is normally established by a POSITIVE voltage on the interface.
There are many different types of interfaces defined. The specific Interface Parameters should be
referred to.
Bit Order
The ORDER OF TRANSMISSION may be established by the protocol and the specific devices
being used. One of the most commonly used methods is to transmit the Least-Significant Bit
(LSB) first and the Parity Bit last, following the Most-Significant Bit (MSB) of data.
Asynchronous Character Format
THE LETTER "A" = (HEX 41) USING ASCII 7 DATA BIT EVEN PARITY

MSB LSB SPACE = + V
___ ___________________ ___
| | | : : : : | | |
<-- S S | 0 | 1 | 0 . 0 . 0 . 0 . 0 | 1 | S | <--- IDLE
___________| |___| . . . . |___| |_____________
^ ^ ^
| | 8 4 2 1 8 4 2 1 | MARK= - V
\__/ |
STOP BITS ' HEX = 4 + 1 `--- START BIT
(1 or 2 BITS) COL + ROW (1 BIT)

NOTE: The order of transmission is from LSB to MSB.
THE LETTER "R" = (HEX D2) USING ASCII 7 DATA BIT EVEN PARITY

MSB LSB SPACE = + V
. . ___ _______ ___ ___
. . | | | : | | : |
<-- S S . 1 . 1 | 0 | 1 | 0 . 0 | 1 | 0 . S | <--- IDLE
___________:___:___| |___| . |___| . |_____________
^ ^ ^
| | 8 4 2 1 8 4 2 1 | MARK= - V
\__/ |
STOP BITS ' HEX = D + 2 `--- START BIT
(1 or 2 BITS) COL + ROW (1 BIT)

NOTE: The order of transmission is from LSB to MSB.
Block Mode Transmission
Characters may be linked together or stored in a Memory Buffer and then transmitted in one
contiguous string where the STOP BIT of one character is immediately followed by the START
BIT of the next character. This contiguous string of characters is referred to as a
TRANSMISSION BLOCK.
The Transmission Block may use special characters to provide control functions and to act as
delimiters to assist in the flow-control and error-recovery procedures. These special characters
are referred to as CONTROL CHARACTERS and normally provide a standard set of controls
and functions.
Some equipment and protocols may modify the use and functions of the Control Characters for
unique circumstances.
Control Characters and Functions
There are many characters that are used for specific functions, the control of the flow of data, the
control of the associated devices, and error reporting. The following table (next three pages) is a
list of the more commonly used CONTROL CHARACTERS and their standard functions.
CONTROL HEX HEX HEX DESCRIPTION OR FUNCTION
CHARACTER 7-E 7-O 8-N
-----------------------------------------------------
NULL = 0-0 1-0 0-0 NULL or PAD character
SOH = 8-1 0-1 0-1 Start Of Header
STX = 8-2 0-2 0-2 Startof TeXt
ETX = 0-3 8-3 0-3 End of TeXt
EOT = 8-4 0-4 0-4 End Of Transmission
ENQ = 0-5 8-5 0-5 ENQuiry
ACK = 0-6 8-6 0-6 ACKnowledgment
BEL = 8-7 0-7 0-7 BELl or alarm character
BS = 8-8 0-8 0-8 Back Space character
HT = 0-9 8-9 0-9 Horizontal Tabulation
LF = 0-A 8-A 0-A Line Feed
VT = 8-B 0-B 0-B Vertical Tabulation
FF = 0-C 8-C 0-C Form Feed or top of form
CR = 8-D 0-D 0-D Carriage Return
SO = 8-E 0-E 0-E Shift Out
SI = 0-F 9-F 0-F Shift In
DLE = 9-0 1-0 1-0 Data Link Escape
DC1 = 1-1 9-1 1-1 Device Control 1 - READER ON
DC2 = 1-2 9-2 1-2 Device Control 2 - PUNCH ON
DC3 = 9-3 1-3 1-3 Device Control 3 - READER OFF
DC4 = 1-4 9-4 1-4 Device Control 4 - PUNCH OFF
NAK = 9-5 1-5 1-5 Negative AcKnowledgment
SYN = 9-6 1-6 1-6 SYNchronizing character (SYNC)
ETB = 1-7 9-7 1-7 End of Transmission Block
CAN = 1-8 9-8 1-8 CANcel
EM = 9-9 1-9 1-9 End of Media
SUB = 9-A 1-A 1-A SUBstitute character
ESC = 1-B 9-B 1-B ESCape character
FS = 9-C 1-C 1-C File Separator
GS = 1-D 9-D 1-D Group Separator
RS = 1-E 9-E 1-E Record Separator
US = 9-F 1-F 1-F Unit Separator
DEL = F-F 7-F F-F DELete or trailing PAD
Transmission Block (Message)
The normal message or transmission block consists of a BEGINNING, the DATA or TEXT, and
an ENDING.
The BEGINNING of a message is indicated by the Start Of Header (SOH) or the Start Of Text
(STX) characters. The header or text will follow the respective characters. The END of the data
or text is indicated by the End of Text (ETX) or the End of Transmission Block (ETB)
characters.
The BLOCK MODE transmission protocol may provide error detection on each character with
the use of character parity, also referred to as VERTICAL REDUNDANCY CHECK (VRC) or
VERTICAL PARITY.
The Block Mode may also include the entire Message in an ERROR DETECTION Procedure
that is referred to as a BLOCK CHECK or LONGITUDINAL REDUNDANCY CHECK (LRC)
also referred to as the HORIZONTAL PARITY.
The CHARACTER PARITY and BLOCK CHECK procedure is designed to ensure that all of
the BITS that are sent by the TRANSMITTING Device are correctly Received by the
RECEIVING Device. There are several algorithms that may be used that may provide different
levels of accuracy or validity.
FORMAT
_____ ________ _____ ______________________ _____
| | | | | |
| SOH | HEADER | STX | TEXT OR DATA MESSAGE | BCC |
|_____|________|_____|______________________|_____|
^ ^
| |
| / This portion of the \ |
`------{ transmission is }-----'
\ protected by the BCC /

Error Detection and Correction
The TRANSMITTING device passes all of the bits of the message through an arithmetic process
that generates a form of CHECK SUMMATION (CHECKSUM or BLOCK CHECK) of all of
the bits and appends the results of the CHECKSUM to the END of the message.
The RECEIVER will perform the same arithmetic process while receiving all of the bits and will
compare the results (CHECKSUM or BLOCK CHECK) of the arithmetic process with the
results included with the message.
If the two BLOCK CHECK factors compare then the message is assumed to be VALID and the
receiving device will respond with an ACK (Positive Acknowledgment) to the message and the
transmitting device may send the NEXT message.
If the two factors or BLOCK CHECKS do not compare, then the message is assumed to contain
errors and the receiving device will NAK (Negative Acknowledgment) the message. The
message must be RETRANSMITTED until the receiving device responds with the ACK or the
RETRY LIMIT is reached.
The number of times that the sending device will try to transmit the message is controlled by the
specific protocol being used. If the error rate is too high the session may be ABORTED.


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