Automation Using PLC
Content
PROJECT REPORT ON AUTOMATION USING POWER LINE COMMUNICATION
SUBMITTED BY:
Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
UNDER GUIDANCE OF
PROF. T.R PAUL
DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION WATUMULL INSTITUTE OF ELECTRONICS ENGINEERING AND COMPUTER TECHNOLOGY UNIVERSITY OF MUMBAI 2009-2010
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PROJECT REPORT ON “AUTOMATION USING POWER LINE COMMUNICATION”
Submitted in partial fulfilment of requirements for the degree course of “BACHELOR OF ENGINEERING IN ELECTRONICS AND TELECOMMUNICATION [EXTC]” By Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
Under the noted guidance of
Prof. (Mr.) T R Paul
University Of Mumbai 2009-2010 DEPARTMENT OF EXTC ENGG. Watumull Institute Of Electronics Engineering And Computer Technology,
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Worli.
ACKNOWLEDGEMENT
We acknowledge with sincere gratitude, the whole hearted support both technically and morally that we have received from our HOD, PROF. (MRS.) SUNITA SHARMA. We would like to thank for the sincere appreciation and encouragement that we have received during the launching of this project from our GUIDE, Mr. T.R PAUL. We also thank the lab technician MISHRA SIR and other non teaching staff of our EXTC department for their help during the development of this project. We are thankful to our colleagues and all those who helped us knowingly or unknowingly to make our project a success. We have realized that the efficiency of team work is indeed more than an individual effort. The guidance of our guide motivated us to work harder under all circumstances.
SUBMITTED BY:
Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
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TABLE OF CONTENS
1.
2.
3.
4. 5. 6. 7. 8. 9.
Introduction. 1.1 Power Line Communication. 1.2 Modern Applications and Prospects. 1.3 Project Aim. Protocols. 2.1 Existing Industry Protocols used in PLC 2.2 X10 2.2.1 Overview. 2.2.2 Protocol. 2.2.3 Sequence of Signals. 2.2.4 Disadvantages. 2.3 MODBUS 2.3.1 Overview. 2.3.2 MODBUS Message Structure. 2.3.3 MODBUS Serial Transmission Modes. 2.3.4 MODBUS Addressing. 2.3.5 MODBUS Function Codes. 2.3.6 Disadvantages. Circuit Design 3.1 Power line as Communication Channel. 3.2 Digital Modulation. 3.3 Transmitter Structure. 3.4 Power Line Interface (Isolation Circuit): 3.5 Receiver Structure. System Implementation - The Transmitter. 4.1 Hardware. 4.2 Software. System Implementation - The Receiver. 5.1 Hardware 5.2 Software Cost of Project. Conclusion. Bibliography Appendix.
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List of Figures
Figure 1: Electric power transmission scheme Figure 2: Power Line Waveform with Zero Crossing for trans-receiving X10 data Figure 3: Receiver Sensing Zero Crossing Figure 5: Start Code. Figure 4: Zero crossing Figure 5: Start Code. Figure 6: Letter Code. Figure 7: Number Code. Figure 8: Transmission of Address Data. Figure 9: Address Code and Command Code separation. Figure 10: Power Line Phases and X-10 pulse. Figure 11: MODBUS message structure. Figure 13: Device and MODBUS address ranges Figure 14: MODBUS Function Codes Figure 15: Power Line Communication Channel Figure 16: Binary Amplitude Shift Keying Waveform Figure 17: A BFSK waveform derived from a Binary message. Figure 18: The Transmitter Circuit diagram Figure 19: The Receiver Circuit Diagram Figure 20: The Power Line Communication System
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Abstract
In this project tilted power line communication we discuss how the existing power line network operating at 230V (50 Hz) can be used for digital data transfer. The transmitter microcontroller is used to generate the digital carrier and the binary codes which control which devices are actuated at the receiving end. The receiver microcontroller receives the binary codes and controls the appropriate devices. The coupling circuitry and the isolation mechanism used at high voltages are also specified. Finally the applications of the project and the future scope of the technology are specified.
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1. INTRODUCTION
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1. INTRODUCTION
Establishing an efficient and rapid means of communication between devices constituting a network has remained the main research initiative of communication engineers for a long time now. The concept initially started with an aim of establishing a communication link between two devices .Slowly the devices constituting the network multiplied until we faced the task of interfacing thousands of devices together to a common communication channel. As the frequencies to be used changed and the cost of establishing network links increased, researchers began the work of finding new media and new technologies which could enable communication in a network at high speeds and low cost. Initially when devices were connected dedicated communication lines were laid down for communication. Miles and miles of wire were laid to enable communication over longer distances. Hence the cost continued to remain a concern. Hence scientists came up with the idea of using already established networks, such as the one carrying AC power to our household, as a communication pathway. This eliminated the need to establish separate dedicated networks and hence resulted in reduced costs. 1.1 Power Line Communication : Power Line Communications (PLC) is the use of existing electrical cables to transport data. It is a scheme in which data is transmitted over a conductor used for electric power transmission. Here a modulated carrier signal is superimposed on the wiring system. Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Power line communications can be applied at each stage.
Figure 1: Electric power transmission scheme 8
Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power wiring system was originally intended for transmission of AC power, in conventional use, the power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications. 1.2 Modern Applications & Prospects Power Line Communications (PLC) has been around for a very long time. In this section we would also like to discuss some major applications driving the Power Line Communication (PLC) technology. They are:
Automatic Meter Reading (AMR) – For the readings of Electricity, Water, Gas or any other meters in the customer premises to be transmitted to a central base station for further processing, billing etc. With tens of millions of meters to be read periodically and regularly, this alone represents an enormous market.
Home automation - Power line communications technology can use the household electrical power wiring as a transmission medium. INSTEON, X10 and MODBUS are the two most popular de facto standards using power line communications for home control. This is a technique used in home automation for remote control of lighting and appliances without installation of additional control wiring.
Distribution Automation, and Supervisory Control and Distribution Automation (DA and SCADA) – This is for the utility companies themselves to monitor and control the Power Distribution Process.
1.3 Project Aim The project aims to thoroughly explore the theoretical and practical aspects of power line communications (PLC) techniques. To this end a number of specific goals were proposed at the start of the project.
To gain a detailed knowledge of the challenges faced by PLC techniques-why they are not a widespread communications method.
To research and design a working PLC system.
To use the design and implement a power line carrier communications system that connects two microprocessor/micro-controller kits as also two personal computers. The devices should be able to transfer data using the power lines as their only link of communication.
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2. PROTOCOLS
2. PROTOCOLS
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When establishing a communication network we first decide what transmission medium is to be used and then we decide the communication protocol to be used. A communications protocol is the set of standard rules for data representation, signaling, authentication and error detection required to send information over a channel. The following are the pre-requisites of a communications protocol:
• Transmission data rate and reception data rate should be same
• Format of the data packet as sent by the transmitter should be understood by the receiver
• Transmitters and receivers should use compatible coding and decoding schemes
respectively • Both the transmitter and receiver should use the same error detection algorithm
2.1 Existing Industry Standards in PLC
Some of the most commonly used automation protocols used in the industry today for wired communication are given below:
• X 10 • MODBUS • C-Bus • KNX • INSTEON We give a detailed description of the X10 and MODBUS protocols along with a comparison of their features in the following section. 2.2 X10 2.2.1 Overview: X10 is an international and open industry standard for communication among electronic devices used for home automation, also known as domotics. It primarily uses power line wiring for signaling and control, where the signals involve brief radio frequency bursts representing digital information.
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Household electrical wiring — the same which powers lights and appliances — is used to send digital data between X10 devices. 2.2.2 Protocol: This digital data is encoded onto a power line carrier which is transmitted as bursts during the relatively quiet zero crossings of the AC alternating current waveform. One bit is transmitted at each zero crossing.
Figure 2: Power Line Waveform with Zero Crossing for trans-receving X10 data
A receiver opens it’s receive "window" twice each sine wave i.e. 120 times each second.
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Figure 3: Receiver Sensing Zero Crossing.
Figure 4: Zero crossing Whether using power line or radio communications, packets transmitted using the X10 control protocol consist of a four bit house code followed by one or more four bit unit code, finally followed by a four bit command. For the convenience of users configuring a system, the four bit house code is selected as a letter from A through P while the four bit unit code is a number 1 through 16. When the system is installed, each controlled device is configured to respond to one of the 256 possible addresses (16 house codes × 16 unit codes); each device reacts to commands specifically addressed to it, or possibly to several broadcast commands. 2.2.3 Sequence of Signals: 1. In order to provide a predictable start point, every data frame would always begin with at least 6 leading clear zero crossings, then a start code of 1110.
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Figure 5: Start Code. 2. Once the start code is sent, a 4 digit letter code is sent(house code).
Figure 6: Letter Code. 3. Then a 4 digit function code is sent(1-16)
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Figure 7: Number Code. Function codes may specify a unit number code (1–16) or a command code, the selection between the two modes being determined by the last bit where 0=unit number and 1=command. One start code, one letter code, and one function code is known as an X10 frame and represent the minimum components of a valid X10 data packet.
4. Each frame is sent twice in succession to make sure the receivers understand it over any power line noise for purposes of redundancy, reliability, and to accommodate line repeaters.
Figure 8: Transmission of Address Data.
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5. Whenever the data changes from one address to another address, from an
address to a command, or from one command to another command, the data frames must be separated by at least 6 clear zero crossings (or "000000"). The sequence of six zeros resets the device decoder hardware.
Figure 9: Address Code and Command Code separation. Till now all the diagrams have shown signals on a single sinusoidal line. But our mains power supply generates electric power in 3 phases. Hence instead of sending just one pulse each transmitter should send three pulses every half sine wave.
Figure 10: Power Line Phases and X-10 pulse. 2.2.4 Disadvantages:
• TVs or wireless devices may cause spurious off or on signals. Noise filtering
(as installed on computers as well as many modern appliances) may help keep external noise out of X10 signals, but noise filters not designed for X10 may
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also filter out X10 signals traveling on the branch circuit to which the appliance is connected.
• X10 signals can only be transmitted one command at a time, first by
addressing the device to control, and then sending an operation for that device to perform. If two X10 signals are transmitted at the same time they may collide or interleave, leading to commands that either cannot be decoded or that trigger incorrect operations.
• The X10 protocol is also slow.
2.3 MODBUS
2.3.1 Overview: The MODBUS communication interface is built around messages. The format of these MODBUS messages is independent of the type of physical interface used. This gives the MODBUS interface definition a very long lifetime. The same protocol can be used regardless of the connection type. Because of this, MODBUS gives the possibility to easily upgrade the hardware structure of an industrial network, without the need for large changes in the software. A device can also communicate with several MODBUS nodes at once, even if they are connected with different interface types, without the need to use a different protocol for every connection. 2.3.2 MODBUS Message Structure: Each MODBUS message has the same structure. Four basic elements are present in each message. The sequence of these elements is the same for all messages, to make it easy to parse the content of the MODBUS message. A conversation is always started by a master in the MODBUS network. A MODBUS master sends a message and—depending of the contents of the message—a slave takes action and responds to it. There can be more masters in a MODBUS network. Addressing in the message header is used to define which device should respond to a message. All other nodes on the MODBUS network ignore the message if the address field doesn't match their own address.
MODBUS message structure Field Description
Device address Address of the receiver
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Function code Code defining message type Data Error check Data block with additional information Numeric check value to test for communication errors Figure 11: MODBUS message structure. 2.3.3 MODBUS serial transmission modes: MODBUS/ASCII and MODBUS/RTU Serial MODBUS connections can use two basic transmission modes, ASCII or Remote Terminal Unit (RTU). The transmission mode in serial communications defines the way the MODBUS messages are coded. With MODBUS/ASCII, the messages are in a readable ASCII format. The MODBUS/RTU format uses binary coding which makes the message unreadable when monitoring, but reduces the size of each message which allows for more data exchange in the same time span. All nodes on one MODBUS network segment must use the same serial transmission mode. A device configured to use MODBUS/ASCII cannot understand messages in MODBUS/RTU and vice versa. When using MODBUS/ASCII, all messages are coded in hexadecimal values, represented with readable ASCII characters. Only the characters0...9 and A...F are used for coding. For every byte of information, two communication-bytes are needed, because every communication-byte can only define 4 bits in the hexadecimal system. With MODBUS/RTU the data is exchanged in a binary format, where each byte of information is coded in one communication-byte. MODBUS messages on serial connections are not sent in a plain format. They are framed to give receivers an easy way to detect the beginning and end of a message. When using MODBUS/ASCII, characters are used to start and end a frame. The colon ':' is used to flag the start of a message and each message is ended with a CR/LF combination. MODBUS/RTU on the other hand uses time gaps of silence on the communication line for the framing. Each message must be preceded by a time gap with a minimum length of 3.5 characters. If a receiver detects a gap of at least 1.5 characters, it assumes that a new message is coming and the receive buffer is cleared. The main advantage of MODBUS/ASCII is, that it allows gaps between the bytes of a message with a maximum length of 1 second. With MODBUS/RTU it is necessary to send each message as a continuous stream.
Properties of MODBUS/ASCII and MODBUS/RTU MODBUS/ASCII
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MODBUS/RTU
Characters Error check Frame start Frame end Gaps in message Start bit Data bits Parity Stop bits
ASCII 0...9 and A..F
Binary 0...255
LRC Longitudinal Redundancy Check CRC Cyclic Redundancy Check character ':' characters CR/LF 1 sec 1 7 even/odd 1 none 2 3.5 chars silence 3.5 chars silence 1.5 times char length 1 8 even/odd 1 none 2
Figure 12: Properties of MODBUS/ASCII and MODBUS/RTU 2.3.4 MODBUS Addressing The first information in each MODBUS message is the address of the receiver. This parameter contains one byte of information. In MODBUS/ASCII it is coded with two hexadecimal characters, in MODBUS/RTU one byte is used. Valid addresses are in the range 0-247. The values 1-247 are assigned to individual MODBUS devices and 0 is used as a broadcast address. Messages sent to the latter address will be accepted by all slaves. A slave always responds to a MODBUS message. When responding it uses the same address as the master in the request. In this way the master can see that the device is actually responding to the request. Within a MODBUS device, the holding registers, inputs and outputs are assigned a number between 1 and 10000. One would expect that the same addresses are used in the MODBUS messages to read or set values. Unfortunately this is not the case. In the MODBUS messages addresses are used with a value between 0 and 9999. If you want to read the value of output (coil) 18 for example, you have to specify the value 17 in the MODBUS query message. More confusing is even, that for input and holding registers an offset must be subtracted from the device address to get the proper address to put in the MODBUS message structure. This leads to common mistakes and should be taken care of when designing applications with MODBUS. The following table shows the address ranges for coils, inputs and holding registers and the way the address in the MODBUS message is calculated given the actual address of the item in the slave device.
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Device and MODBUS address ranges Device address MODBUS address Description Coils (outputs) Inputs Holding registers
1...10000* address – 1 10001...20000* address - 10001 40001...50000* address - 40001
*
Maximum value is device dependent
Figure 13: Device and MODBUS address ranges 2.3.5 MODBUS Function Codes The second parameter in each MODBUS message is the function code. This defines the message type and the type of action required by the slave. The parameter contains one byte of information. In MODBUS/ASCII this is coded with two hexadecimal characters, in MODBUS/RTU one byte is used. Valid function codes are in the range 1..255. Not all MODBUS devices recognize the same set of function codes. The most common codes are discussed here. Normally, when a MODBUS slave answers a response, it uses the same function code as in the request. However, when an error is detected, the highest bit of the function code is turned on. In that way the master can see the difference between success and failure responses.
Common MODBUS function codes Code 01 Description Read coil status
02 03
Read input status Read holding registers
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04 05 06 07 15 16 17
Read input registers Force single coil Preset single register Read exception status Force multiple coils Preset multiple registers Report slave ID
Figure 14: MODBUS Function Codes
2.3.6 Disadvantages:
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MODBUS was designed in the late 1970s to communicate to programmable logic controller’s the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported. Since MODBUS is a master/slave protocol, there is no way for a field device to "report by exception"-the master node must routinely poll each field device, and look for changes in the data. This consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link. MODBUS is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station.
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3. CIRCUIT DESIGN
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3. CIRCUIT DESIGN
As stated earlier, the project aims at developing a system that uses the AC power lines as the communication channel/medium to transmit and receive information. In this chapter we shall study the various circuits that form the basis of the power line communication system. We would come across the actual system implementation in the next chapter. Let us first start with describing the power line as a communication channel
3.1 The Communication Channel: The low-voltage power grid that is used to supply electric power to houses consists of many channels each with its own characteristics and quality. Figure below shows a communication system using the power-line as a communication channel. The transmitter is shown to the left and the receiver to the right. Important parameters of the communication system are the output impedance, Zt, of the transmitter and the input impedance, Zi, of the receiver.
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Figure 15: Power Line Communication Channel
3.2 Transmitter: The transmitter in the PLC system provides the necessary signal which is to be transmitted through the channel. The signal generated is digital in nature (Binary 0’s & 1’s or a sequence of 0’s n 1’s). A microcontroller is programmed by software to generate the necessary digital signal.
The transmitter needs to make sure it provides the necessary signal without being affected by external parameters. The digital signal is modulated in proper form to make the signal more noise free, efficient, and immune to stray signals. Let us now study the various digital modulation schemes.
3.3 Modulation Schemes: The transmitter (a Microcontroller in our case) is the source of digital signal for communication purpose. The source signals are generally referred to as Base-band signals. The low-frequency signal is often frequency-translated to a higher frequency range for efficient transmission. The process is called modulation. In the modulation process, the base-band signals constitute the modulating signal and the highfrequency carrier signal is a sinusoidal waveform. There are three basic ways of modulating a sine wave carrier. They are
Binary Amplitude-Shift keying (BASK). Binary Frequency-Shift keying (BFSK). Binary Phase-Shift Keying (BPSK).
3.2.1 Binary Amplitude shift keying (BASK) - In the context of digital communications, BASK is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels adopted by the digital message. Mathematically it can be expressed as v(t) = A* b(t) * cos(wo t) Where, A = amplitude wo = carrier frequency b(t ) = Uni-polar digital data
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When b(t ) = ’1’ When b(t ) = ‘0’
v(t) = A* cos (wo t) v(t) = 0
This implies that the carrier frequency is present when data is at high logic level and there is no carrier at all when the data is at logic low level. For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated waveform consists of bursts of a sinusoid. Figure illustrates a binary ASK signal, together with the binary sequence which initiated it
Figure 16: Binary Amplitude Shift Keying Waveform
3.2.2 Binary Frequency Shift Keying (BFSK): It’s the most common form of digital modulation in the high-frequency radio spectrum. Binary FSK (usually referred to simply as FSK) is a modulation scheme typically used to send digital information between digital equipment such as tele-printers and computers. Data is transmitted by shifting the frequency of a continuous carrier in a binary manner to one or the other of two discrete frequencies. Mathematically it can be expressed as v(t) = A* cos[wot + d (t ) *W* t] Where A = amplitude wo = center frequency d (t) = +1 for logic level ‘1’ Thus the transmitted signal is either v(t) = A* cos[wo + W] * t or v(t) = A* cos[wo - W]* t
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W = frequency deviation d (t) = -1 for logic level ‘0’
And hence two analog waves of different frequencies are obtained. The waveforms for BFSK are shown below.
Figure 17: A BFSK waveform derived from a Binary message.
3.3. Power Line Interface (Isolation Circuit): One of the most important parts of our power line transmitter and receiver is the Power Line Interface. Because our circuit has to connect to the 230V, 50 Hz power line, without careful isolation, the rest of the circuit will be easily damaged. The idea is to superimpose the data signal onto the 230V, 50 Hz power waveform, and extract it afterwards at the receiving end. The ideal isolation circuit should completely block the 50Hz signal, and pass the information signal. The information signal in our case is the frequency modulated signal.
3.4. Receiver: The signal generated by the transmitted is sufficiently amplified and passed through the isolation and coupling circuitry onto the A.C power lines. The high frequency digital signal gets superimposed over the low (50Hz) power lines. The signal thus passes over the power lines. This superimposed signal reaches the receiver. The receiver is designed to perform the following actions. Filtering and Isolation: The received signal is passed through a high pass (RC) filter which is designed specifically to filter out the 50 Hz power line signal. The powerful A.C signal should be prevented from entering the receiver peripherals as it can cause damage to the components. Thus the foremost function of the receiver must be to filter out this signal. Thereafter careful isolation from the 230V must be provided to the receiver components as it can also cause sufficient damage. At the output of this portion of the receiver circuit we get the transmitted high frequency digital signal.
Wave Shaping: The digital signal recovered from the transmitted signal can be affected by noise introduced during its travel through the power lines. Any stray noise pulses can cause unwanted action at the receiver. Therefore the receiver should wave shape the recovered digital signal to take care of the unwanted stray pulses.
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Control Action: The recovered digital signal is then used by the microcontroller in the receiver to perform the necessary control action on the devices connected to the receiver. The receiver should thus differentiate between the different signals for the respective devices and the take the necessary controlling action.
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4. System Implementation: The Transmitter
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4. System Implementation: The Transmitter
The chapters discussed earlier described the various design methodologies used in building a power line communication system. We now study the actual circuit of the transmitter and the receiver of the Power Line Communication system in chapter 4 & 5.
Let us start with the transmitter.
The basic layout of the transmitter circuit diagram is shown below.
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Figure 18: The Transmitter Circuit diagram
4.1 Hardware:
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The heart of the transmitter is the microcontroller. The system deploys the 8052 microcontroller because the timer2 in the 8052 can be programmed to generate a fixed frequency digital pulse train which can be used as a carrier in the amplitude shift keying modulation process. The microcontroller is programmed to output a frequency of 100 KHz at pin P1.0. The output from this pin is AND’ed with the signal from the transmit pin (TX) of the microcontroller. The output of the AND gate gives the amplitude modulated waveform. This waveform is subsequently fed to a transistor amplifier which amplifies the resultant waveform and also couples it to the input side of the opto-coupler circuitry.
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The opto-coupler is used for the isolation of the microcontroller and other allied low voltage circuitry from the high voltage power line. The output of the optocoupler is capacitor coupled to the high voltage power line as shown in the figure.
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4.2 Software
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The transmitter microcontroller performs two main functions 1. Generating the carrier digital pulse train 2. Transmitting data bytes to control appliance at the receiver end.
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The carrier frequency of 100 KHz is generated using the Timer2 of the 8052 microcontroller in the “Programmable Clock Out “mode. Here the timer2 is used to generate a continuous pulse train of 50% duty cycle having frequency of 100 KHz on pin P1.0.
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We have programmed the microcontroller on the transmitter side to send four different bytes depending on the action to be performed.
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Byte 1: Turns led 1 on Byte 2: Turns led 1 off
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Byte 3: Turns led 2 on Byte 4: Turns led 2 off
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These bytes are sent depending on the status of the pins 2.0, 2.1, 2.2, 2.3. The status of these pins is manipulated using four switches connected to these pins.
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89C52 Programming for the Transmitter Circuit
include reg_52.pdf
cseg
at SJMP
00h MAIN 03H AJMP ZCD_INT 23h AJMP UART_INT
CSEG
AT
cseg
at
BSEG SEND_DATA_NOW: KEY_DTCT_1: KEY_DTCT_2: KEY_DTCT_3: KEY_DTCT_4:
AT DBIT DBIT DBIT DBIT DBIT
0 01 01 01 01 01
DSEG
AT
25H
DST_REP_COUNT:
DS
01
KEY1 KEY2 KEY3 KEY4
EQU EQU EQU EQU
P2.0 ; Clock pin assignment for I2C P2.1 ; Clock pin assignment for I2C P2.2 ; Clock pin assignment for I2C P2.3 ; Clock pin assignment for I2C
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CSEG MAIN: MOV MOV MOV MOV MOV MOV SETB SETB SETB SETB SETB MOV MOV
AT
30H
T2CON,#04H T2MOD,#02H SCON,#50h TMOD,#20h TH1,#0f4h TL1,#0f4h TR1 ES EX0 IT0 EA RCAP2H,#0FFH RCAP2L,#0E2 ;2400 Baud-rate
HERE: JBC JB JB SETB MOV SEND_DATA_NOW,SEND_DATA KEY_DTCT_1,CHK_KEY2 KEY1,CHK_KEY2 KEY_DTCT_1 DST_REP_COUNT,#2
CHK_KEY2: JB KEY_DTCT_2,CHK_KEY3
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JB SETB MOV
KEY2,CHK_KEY3 KEY_DTCT_2 DST_REP_COUNT,#2
CHK_KEY3: JB JB SETB MOV KEY_DTCT_3,CHK_KEY4 KEY3,CHK_KEY4 KEY_DTCT_3 DST_REP_COUNT,#2
CHK_KEY4: JB JB SETB MOV SJMP KEY_DTCT_4,HERE KEY4,HERE KEY_DTCT_4 DST_REP_COUNT,#2 HERE
SEND_DATA: JB JB JB JB SJMP HERE KEY_DTCT_1,SEND_55 KEY_DTCT_2,SEND_AA KEY_DTCT_3,SEND_5A KEY_DTCT_4,SEND_A5
SEND_55: MOV SBUF,#55H
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DJNZ CLR SJMP
DST_REP_COUNT,HERE KEY_DTCT_1 HERE
SEND_AA: MOV DJNZ CLR SJMP SBUF,#0AAH DST_REP_COUNT,HERE KEY_DTCT_2 HERE
SEND_5A: MOV DJNZ CLR SJMP SBUF,#5AH DST_REP_COUNT,HERE KEY_DTCT_3 HERE
SEND_A5: MOV DJNZ CLR SJMP SBUF,#0A5H DST_REP_COUNT,HERE KEY_DTCT_4 HERE
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; UART_INT:
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JBC JBC
TI,EXIT_UART RI,EXIT_UART
EXIT_UART: RETI
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ZCD_INT:
SETB SEND_DATA_NOW RETI END
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5. System Implementation: The Receiver
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5. System Implementation: The Receiver
We studied the transmitter in detail in the previous chapter. Now we shall look closer into the receiver of the power line communication system in this chapter. The signal received from the transmitter needs to be recovered and the necessary action is to be taken on the devices connected to the receiver.
The circuit layout of the receiver is given below.
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Figure 19: The Receiver circuit Diagram 5.1 Hardware:
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The input at the receiving end is a 50 Hz power line signal superimposed with the 100 hz signal frequency. This composite signal is passed through a high pass RC filter to remove the 50 Hz component and pass the 100 KHz component(data signal). The signal is then amplified and fed to the input of the opto-coupler. The optocoupler isolates the low voltage microcontroller section from the high voltage section consisting of the power line. The output of the opto-coupler is again amplified and fed to the microcontroller where necessary processing takes place.
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5.2 Software
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The receiver microcontroller performs two main functions 1. Wave shaping of the received signal to eliminate the effect of noise transients and for signal smoothening 2. Switching the devices connected to the pins based on the code received.
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The received signal as a result is fed at the external interrupt 1. The interrupt subroutine performs the necessary wave shaping and gives the wave shaped output on pin P1.3. Pin P1.3 is then connected to the receive (RX) pin of the microcontroller. Depending on the byte received the microcontroller switched the LED’s connected to the pin P2.0 and pin P2.1.
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89C52 Programming for the Receiver Circuit
include reg_52.pdf
cseg
at SJMP
00h MAIN 13H SJMP EXT_1 0BH SJMP TMR_0 23h SJMP UART_INT
CSEG
AT
CSEG
AT
cseg
at
DSEG SR_DATA:
AT DS
25H 01
BSEG F_T_FLAG: VALID_DATA:
AT DBIT DBIT
00 01 01
OP1 OP2
EQU EQU
P2.0 P2.1
; Clock pin assignment for I2C ; Clock pin assignment for I2C
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CSEG
AT
30H
MAIN: CLR CLR CLR MOV MOV MOV MOV SETB SETB SETB SETB SETB SETB MOV SETB OP1 OP2 P1.3 SCON,#50h TMOD,#22h TH1,#0f4h TL1,#0f4h TR1 ES ET0 EX1 TR0 IT1 IP,#04H EA ;2400 Baud-rate
HERE: JBC SJMP VALID_DATA,PROC_SR_DATA HERE
PROC_SR_DATA: MOV CJNE A,SR_DATA A,#55H,CHK_4_AA
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SETB SJMP
OP1 HERE
CHK_4_AA: CJNE CLR SJMP A,#0AAH,CHK_4_5A OP1 HERE
CHK_4_5A: CJNE SETB SJMP A,#5AH,CHK_4_A5 OP2 HERE
CHK_4_A5: CJNE CLR SJMP A,#0A5H,HERE OP2 HERE
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; EXT_1: JB SETB SJMP F_T_FLAG,CONT_EXT_1 F_T_FLAG CONT_EXT_2
CONT_EXT_1: SETB P1.3
49
CONT_EXT_2: MOV SETB RETI TL0,#220 TR0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; TMR_0: CLR CLR CLR RETI P1.3 F_T_FLAG TR0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; UART_INT: JBC JBC TI,EXIT_UART RI,CHK_RCV_DATA
EXIT_UART: RETI
CHK_RCV_DATA:
50
MOV CJNE RETI
A,SBUF A,#0,CHK_4_FF
CHK_4_FF: CJNE RETI A,#0FFH,GO_VALID_DATA
GO_VALID_DATA: MOV SETB RETI ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; SR_DATA,A VALID_DATA
51
5.3 THE POWER LINE COMMUNICATION SYSTEM
52
6.
Cost of Project
53
Sr no: 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
Components PCB Microcontrollers 89C52 MCT2E PC817 Box caps BC548 LED LM2940 Capacitor 1000uF Capacitor 220uF Capacitor 0.1uF Capacitor 0.33pF Crystal Resistor Network Bridge rectifier Capacitor 10uF Bus ship Slide Switch Sockets AND gate Adapter Resistors Switches Transformers Diode Wires
Price(Rs) 50 40 7.5 3 7 6 1 35 5 3 .25 .25 3 3 5 2 4 3 6 8 100 1 4 200 .25 1
Quantity 2 2 2 3 3 1 2 2 2 2 3 4 2 3 2 2 2 2 3 1 3 10 4 1 1 20
Total 100 80 15 9 21 6 2 70 10 6 .75 1 6 9 10 4 8 6 18 8 300 10 16 200 .25 20
Grand Total
Rs.918
7. Conclusion
54
The devices connected to the designed power line communication system can be increased by making minute changes in the underlying hardware and software. The system can be used for a number of applications such as home automation, automatic meter reading etc. The implications of power line communication are farfetched and varied. The technology can be used for controlling appliances in a household as well as for remotely controlling vast industrial establishments. The technology is highly cost effective since it uses the already established power line network for data communications. In spite of its numerous advantages, the communication is limited to low data rates and hampered by high power line noise. Research advances in these areas will further lead to improvements in the technology.
55
8. Bibliography
www.hometoys.com www.lammertbies.nl. Design of Power-Line Communication System (PLC) Using a PIC Microcontroller, Q. Al-Zobi, I. Al-Tawil, K. Gharaibeh and I. S. Al-Kofahi. ITU Press Release: New global standard for fully networked home. IEEE P1901 Press Release. Modicon MODBUS Protocol Reference Guide. Tech note-X-10 Communications Protocol and Power Line Interface.
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57
9. APPENDIX
58
59
60
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66
67
68
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SUBMITTED BY:
Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
UNDER GUIDANCE OF
PROF. T.R PAUL
DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION WATUMULL INSTITUTE OF ELECTRONICS ENGINEERING AND COMPUTER TECHNOLOGY UNIVERSITY OF MUMBAI 2009-2010
1
PROJECT REPORT ON “AUTOMATION USING POWER LINE COMMUNICATION”
Submitted in partial fulfilment of requirements for the degree course of “BACHELOR OF ENGINEERING IN ELECTRONICS AND TELECOMMUNICATION [EXTC]” By Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
Under the noted guidance of
Prof. (Mr.) T R Paul
University Of Mumbai 2009-2010 DEPARTMENT OF EXTC ENGG. Watumull Institute Of Electronics Engineering And Computer Technology,
2
Worli.
ACKNOWLEDGEMENT
We acknowledge with sincere gratitude, the whole hearted support both technically and morally that we have received from our HOD, PROF. (MRS.) SUNITA SHARMA. We would like to thank for the sincere appreciation and encouragement that we have received during the launching of this project from our GUIDE, Mr. T.R PAUL. We also thank the lab technician MISHRA SIR and other non teaching staff of our EXTC department for their help during the development of this project. We are thankful to our colleagues and all those who helped us knowingly or unknowingly to make our project a success. We have realized that the efficiency of team work is indeed more than an individual effort. The guidance of our guide motivated us to work harder under all circumstances.
SUBMITTED BY:
Prabhakaran Ashwin V Sadhnani Tarun I Shah Aesha A Thakrani Manish G
3
TABLE OF CONTENS
1.
2.
3.
4. 5. 6. 7. 8. 9.
Introduction. 1.1 Power Line Communication. 1.2 Modern Applications and Prospects. 1.3 Project Aim. Protocols. 2.1 Existing Industry Protocols used in PLC 2.2 X10 2.2.1 Overview. 2.2.2 Protocol. 2.2.3 Sequence of Signals. 2.2.4 Disadvantages. 2.3 MODBUS 2.3.1 Overview. 2.3.2 MODBUS Message Structure. 2.3.3 MODBUS Serial Transmission Modes. 2.3.4 MODBUS Addressing. 2.3.5 MODBUS Function Codes. 2.3.6 Disadvantages. Circuit Design 3.1 Power line as Communication Channel. 3.2 Digital Modulation. 3.3 Transmitter Structure. 3.4 Power Line Interface (Isolation Circuit): 3.5 Receiver Structure. System Implementation - The Transmitter. 4.1 Hardware. 4.2 Software. System Implementation - The Receiver. 5.1 Hardware 5.2 Software Cost of Project. Conclusion. Bibliography Appendix.
8
11
22
26 34 42 43 44 45
4
List of Figures
Figure 1: Electric power transmission scheme Figure 2: Power Line Waveform with Zero Crossing for trans-receiving X10 data Figure 3: Receiver Sensing Zero Crossing Figure 5: Start Code. Figure 4: Zero crossing Figure 5: Start Code. Figure 6: Letter Code. Figure 7: Number Code. Figure 8: Transmission of Address Data. Figure 9: Address Code and Command Code separation. Figure 10: Power Line Phases and X-10 pulse. Figure 11: MODBUS message structure. Figure 13: Device and MODBUS address ranges Figure 14: MODBUS Function Codes Figure 15: Power Line Communication Channel Figure 16: Binary Amplitude Shift Keying Waveform Figure 17: A BFSK waveform derived from a Binary message. Figure 18: The Transmitter Circuit diagram Figure 19: The Receiver Circuit Diagram Figure 20: The Power Line Communication System
5
Abstract
In this project tilted power line communication we discuss how the existing power line network operating at 230V (50 Hz) can be used for digital data transfer. The transmitter microcontroller is used to generate the digital carrier and the binary codes which control which devices are actuated at the receiving end. The receiver microcontroller receives the binary codes and controls the appropriate devices. The coupling circuitry and the isolation mechanism used at high voltages are also specified. Finally the applications of the project and the future scope of the technology are specified.
6
1. INTRODUCTION
7
1. INTRODUCTION
Establishing an efficient and rapid means of communication between devices constituting a network has remained the main research initiative of communication engineers for a long time now. The concept initially started with an aim of establishing a communication link between two devices .Slowly the devices constituting the network multiplied until we faced the task of interfacing thousands of devices together to a common communication channel. As the frequencies to be used changed and the cost of establishing network links increased, researchers began the work of finding new media and new technologies which could enable communication in a network at high speeds and low cost. Initially when devices were connected dedicated communication lines were laid down for communication. Miles and miles of wire were laid to enable communication over longer distances. Hence the cost continued to remain a concern. Hence scientists came up with the idea of using already established networks, such as the one carrying AC power to our household, as a communication pathway. This eliminated the need to establish separate dedicated networks and hence resulted in reduced costs. 1.1 Power Line Communication : Power Line Communications (PLC) is the use of existing electrical cables to transport data. It is a scheme in which data is transmitted over a conductor used for electric power transmission. Here a modulated carrier signal is superimposed on the wiring system. Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Power line communications can be applied at each stage.
Figure 1: Electric power transmission scheme 8
Different types of power line communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power wiring system was originally intended for transmission of AC power, in conventional use, the power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications. 1.2 Modern Applications & Prospects Power Line Communications (PLC) has been around for a very long time. In this section we would also like to discuss some major applications driving the Power Line Communication (PLC) technology. They are:
Automatic Meter Reading (AMR) – For the readings of Electricity, Water, Gas or any other meters in the customer premises to be transmitted to a central base station for further processing, billing etc. With tens of millions of meters to be read periodically and regularly, this alone represents an enormous market.
Home automation - Power line communications technology can use the household electrical power wiring as a transmission medium. INSTEON, X10 and MODBUS are the two most popular de facto standards using power line communications for home control. This is a technique used in home automation for remote control of lighting and appliances without installation of additional control wiring.
Distribution Automation, and Supervisory Control and Distribution Automation (DA and SCADA) – This is for the utility companies themselves to monitor and control the Power Distribution Process.
1.3 Project Aim The project aims to thoroughly explore the theoretical and practical aspects of power line communications (PLC) techniques. To this end a number of specific goals were proposed at the start of the project.
To gain a detailed knowledge of the challenges faced by PLC techniques-why they are not a widespread communications method.
To research and design a working PLC system.
To use the design and implement a power line carrier communications system that connects two microprocessor/micro-controller kits as also two personal computers. The devices should be able to transfer data using the power lines as their only link of communication.
9
2. PROTOCOLS
2. PROTOCOLS
10
When establishing a communication network we first decide what transmission medium is to be used and then we decide the communication protocol to be used. A communications protocol is the set of standard rules for data representation, signaling, authentication and error detection required to send information over a channel. The following are the pre-requisites of a communications protocol:
• Transmission data rate and reception data rate should be same
• Format of the data packet as sent by the transmitter should be understood by the receiver
• Transmitters and receivers should use compatible coding and decoding schemes
respectively • Both the transmitter and receiver should use the same error detection algorithm
2.1 Existing Industry Standards in PLC
Some of the most commonly used automation protocols used in the industry today for wired communication are given below:
• X 10 • MODBUS • C-Bus • KNX • INSTEON We give a detailed description of the X10 and MODBUS protocols along with a comparison of their features in the following section. 2.2 X10 2.2.1 Overview: X10 is an international and open industry standard for communication among electronic devices used for home automation, also known as domotics. It primarily uses power line wiring for signaling and control, where the signals involve brief radio frequency bursts representing digital information.
11
Household electrical wiring — the same which powers lights and appliances — is used to send digital data between X10 devices. 2.2.2 Protocol: This digital data is encoded onto a power line carrier which is transmitted as bursts during the relatively quiet zero crossings of the AC alternating current waveform. One bit is transmitted at each zero crossing.
Figure 2: Power Line Waveform with Zero Crossing for trans-receving X10 data
A receiver opens it’s receive "window" twice each sine wave i.e. 120 times each second.
12
Figure 3: Receiver Sensing Zero Crossing.
Figure 4: Zero crossing Whether using power line or radio communications, packets transmitted using the X10 control protocol consist of a four bit house code followed by one or more four bit unit code, finally followed by a four bit command. For the convenience of users configuring a system, the four bit house code is selected as a letter from A through P while the four bit unit code is a number 1 through 16. When the system is installed, each controlled device is configured to respond to one of the 256 possible addresses (16 house codes × 16 unit codes); each device reacts to commands specifically addressed to it, or possibly to several broadcast commands. 2.2.3 Sequence of Signals: 1. In order to provide a predictable start point, every data frame would always begin with at least 6 leading clear zero crossings, then a start code of 1110.
13
Figure 5: Start Code. 2. Once the start code is sent, a 4 digit letter code is sent(house code).
Figure 6: Letter Code. 3. Then a 4 digit function code is sent(1-16)
14
Figure 7: Number Code. Function codes may specify a unit number code (1–16) or a command code, the selection between the two modes being determined by the last bit where 0=unit number and 1=command. One start code, one letter code, and one function code is known as an X10 frame and represent the minimum components of a valid X10 data packet.
4. Each frame is sent twice in succession to make sure the receivers understand it over any power line noise for purposes of redundancy, reliability, and to accommodate line repeaters.
Figure 8: Transmission of Address Data.
15
5. Whenever the data changes from one address to another address, from an
address to a command, or from one command to another command, the data frames must be separated by at least 6 clear zero crossings (or "000000"). The sequence of six zeros resets the device decoder hardware.
Figure 9: Address Code and Command Code separation. Till now all the diagrams have shown signals on a single sinusoidal line. But our mains power supply generates electric power in 3 phases. Hence instead of sending just one pulse each transmitter should send three pulses every half sine wave.
Figure 10: Power Line Phases and X-10 pulse. 2.2.4 Disadvantages:
• TVs or wireless devices may cause spurious off or on signals. Noise filtering
(as installed on computers as well as many modern appliances) may help keep external noise out of X10 signals, but noise filters not designed for X10 may
16
also filter out X10 signals traveling on the branch circuit to which the appliance is connected.
• X10 signals can only be transmitted one command at a time, first by
addressing the device to control, and then sending an operation for that device to perform. If two X10 signals are transmitted at the same time they may collide or interleave, leading to commands that either cannot be decoded or that trigger incorrect operations.
• The X10 protocol is also slow.
2.3 MODBUS
2.3.1 Overview: The MODBUS communication interface is built around messages. The format of these MODBUS messages is independent of the type of physical interface used. This gives the MODBUS interface definition a very long lifetime. The same protocol can be used regardless of the connection type. Because of this, MODBUS gives the possibility to easily upgrade the hardware structure of an industrial network, without the need for large changes in the software. A device can also communicate with several MODBUS nodes at once, even if they are connected with different interface types, without the need to use a different protocol for every connection. 2.3.2 MODBUS Message Structure: Each MODBUS message has the same structure. Four basic elements are present in each message. The sequence of these elements is the same for all messages, to make it easy to parse the content of the MODBUS message. A conversation is always started by a master in the MODBUS network. A MODBUS master sends a message and—depending of the contents of the message—a slave takes action and responds to it. There can be more masters in a MODBUS network. Addressing in the message header is used to define which device should respond to a message. All other nodes on the MODBUS network ignore the message if the address field doesn't match their own address.
MODBUS message structure Field Description
Device address Address of the receiver
17
Function code Code defining message type Data Error check Data block with additional information Numeric check value to test for communication errors Figure 11: MODBUS message structure. 2.3.3 MODBUS serial transmission modes: MODBUS/ASCII and MODBUS/RTU Serial MODBUS connections can use two basic transmission modes, ASCII or Remote Terminal Unit (RTU). The transmission mode in serial communications defines the way the MODBUS messages are coded. With MODBUS/ASCII, the messages are in a readable ASCII format. The MODBUS/RTU format uses binary coding which makes the message unreadable when monitoring, but reduces the size of each message which allows for more data exchange in the same time span. All nodes on one MODBUS network segment must use the same serial transmission mode. A device configured to use MODBUS/ASCII cannot understand messages in MODBUS/RTU and vice versa. When using MODBUS/ASCII, all messages are coded in hexadecimal values, represented with readable ASCII characters. Only the characters0...9 and A...F are used for coding. For every byte of information, two communication-bytes are needed, because every communication-byte can only define 4 bits in the hexadecimal system. With MODBUS/RTU the data is exchanged in a binary format, where each byte of information is coded in one communication-byte. MODBUS messages on serial connections are not sent in a plain format. They are framed to give receivers an easy way to detect the beginning and end of a message. When using MODBUS/ASCII, characters are used to start and end a frame. The colon ':' is used to flag the start of a message and each message is ended with a CR/LF combination. MODBUS/RTU on the other hand uses time gaps of silence on the communication line for the framing. Each message must be preceded by a time gap with a minimum length of 3.5 characters. If a receiver detects a gap of at least 1.5 characters, it assumes that a new message is coming and the receive buffer is cleared. The main advantage of MODBUS/ASCII is, that it allows gaps between the bytes of a message with a maximum length of 1 second. With MODBUS/RTU it is necessary to send each message as a continuous stream.
Properties of MODBUS/ASCII and MODBUS/RTU MODBUS/ASCII
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MODBUS/RTU
Characters Error check Frame start Frame end Gaps in message Start bit Data bits Parity Stop bits
ASCII 0...9 and A..F
Binary 0...255
LRC Longitudinal Redundancy Check CRC Cyclic Redundancy Check character ':' characters CR/LF 1 sec 1 7 even/odd 1 none 2 3.5 chars silence 3.5 chars silence 1.5 times char length 1 8 even/odd 1 none 2
Figure 12: Properties of MODBUS/ASCII and MODBUS/RTU 2.3.4 MODBUS Addressing The first information in each MODBUS message is the address of the receiver. This parameter contains one byte of information. In MODBUS/ASCII it is coded with two hexadecimal characters, in MODBUS/RTU one byte is used. Valid addresses are in the range 0-247. The values 1-247 are assigned to individual MODBUS devices and 0 is used as a broadcast address. Messages sent to the latter address will be accepted by all slaves. A slave always responds to a MODBUS message. When responding it uses the same address as the master in the request. In this way the master can see that the device is actually responding to the request. Within a MODBUS device, the holding registers, inputs and outputs are assigned a number between 1 and 10000. One would expect that the same addresses are used in the MODBUS messages to read or set values. Unfortunately this is not the case. In the MODBUS messages addresses are used with a value between 0 and 9999. If you want to read the value of output (coil) 18 for example, you have to specify the value 17 in the MODBUS query message. More confusing is even, that for input and holding registers an offset must be subtracted from the device address to get the proper address to put in the MODBUS message structure. This leads to common mistakes and should be taken care of when designing applications with MODBUS. The following table shows the address ranges for coils, inputs and holding registers and the way the address in the MODBUS message is calculated given the actual address of the item in the slave device.
19
Device and MODBUS address ranges Device address MODBUS address Description Coils (outputs) Inputs Holding registers
1...10000* address – 1 10001...20000* address - 10001 40001...50000* address - 40001
*
Maximum value is device dependent
Figure 13: Device and MODBUS address ranges 2.3.5 MODBUS Function Codes The second parameter in each MODBUS message is the function code. This defines the message type and the type of action required by the slave. The parameter contains one byte of information. In MODBUS/ASCII this is coded with two hexadecimal characters, in MODBUS/RTU one byte is used. Valid function codes are in the range 1..255. Not all MODBUS devices recognize the same set of function codes. The most common codes are discussed here. Normally, when a MODBUS slave answers a response, it uses the same function code as in the request. However, when an error is detected, the highest bit of the function code is turned on. In that way the master can see the difference between success and failure responses.
Common MODBUS function codes Code 01 Description Read coil status
02 03
Read input status Read holding registers
20
04 05 06 07 15 16 17
Read input registers Force single coil Preset single register Read exception status Force multiple coils Preset multiple registers Report slave ID
Figure 14: MODBUS Function Codes
2.3.6 Disadvantages:
•
MODBUS was designed in the late 1970s to communicate to programmable logic controller’s the number of data types is limited to those understood by PLCs at the time. Large binary objects are not supported. Since MODBUS is a master/slave protocol, there is no way for a field device to "report by exception"-the master node must routinely poll each field device, and look for changes in the data. This consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link. MODBUS is restricted to addressing 247 devices on one data link, which limits the number of field devices that may be connected to a master station.
•
•
21
3. CIRCUIT DESIGN
22
3. CIRCUIT DESIGN
As stated earlier, the project aims at developing a system that uses the AC power lines as the communication channel/medium to transmit and receive information. In this chapter we shall study the various circuits that form the basis of the power line communication system. We would come across the actual system implementation in the next chapter. Let us first start with describing the power line as a communication channel
3.1 The Communication Channel: The low-voltage power grid that is used to supply electric power to houses consists of many channels each with its own characteristics and quality. Figure below shows a communication system using the power-line as a communication channel. The transmitter is shown to the left and the receiver to the right. Important parameters of the communication system are the output impedance, Zt, of the transmitter and the input impedance, Zi, of the receiver.
23
Figure 15: Power Line Communication Channel
3.2 Transmitter: The transmitter in the PLC system provides the necessary signal which is to be transmitted through the channel. The signal generated is digital in nature (Binary 0’s & 1’s or a sequence of 0’s n 1’s). A microcontroller is programmed by software to generate the necessary digital signal.
The transmitter needs to make sure it provides the necessary signal without being affected by external parameters. The digital signal is modulated in proper form to make the signal more noise free, efficient, and immune to stray signals. Let us now study the various digital modulation schemes.
3.3 Modulation Schemes: The transmitter (a Microcontroller in our case) is the source of digital signal for communication purpose. The source signals are generally referred to as Base-band signals. The low-frequency signal is often frequency-translated to a higher frequency range for efficient transmission. The process is called modulation. In the modulation process, the base-band signals constitute the modulating signal and the highfrequency carrier signal is a sinusoidal waveform. There are three basic ways of modulating a sine wave carrier. They are
Binary Amplitude-Shift keying (BASK). Binary Frequency-Shift keying (BFSK). Binary Phase-Shift Keying (BPSK).
3.2.1 Binary Amplitude shift keying (BASK) - In the context of digital communications, BASK is a modulation process, which imparts to a sinusoid two or more discrete amplitude levels. These are related to the number of levels adopted by the digital message. Mathematically it can be expressed as v(t) = A* b(t) * cos(wo t) Where, A = amplitude wo = carrier frequency b(t ) = Uni-polar digital data
24
When b(t ) = ’1’ When b(t ) = ‘0’
v(t) = A* cos (wo t) v(t) = 0
This implies that the carrier frequency is present when data is at high logic level and there is no carrier at all when the data is at logic low level. For a binary message sequence there are two levels, one of which is typically zero. Thus the modulated waveform consists of bursts of a sinusoid. Figure illustrates a binary ASK signal, together with the binary sequence which initiated it
Figure 16: Binary Amplitude Shift Keying Waveform
3.2.2 Binary Frequency Shift Keying (BFSK): It’s the most common form of digital modulation in the high-frequency radio spectrum. Binary FSK (usually referred to simply as FSK) is a modulation scheme typically used to send digital information between digital equipment such as tele-printers and computers. Data is transmitted by shifting the frequency of a continuous carrier in a binary manner to one or the other of two discrete frequencies. Mathematically it can be expressed as v(t) = A* cos[wot + d (t ) *W* t] Where A = amplitude wo = center frequency d (t) = +1 for logic level ‘1’ Thus the transmitted signal is either v(t) = A* cos[wo + W] * t or v(t) = A* cos[wo - W]* t
25
W = frequency deviation d (t) = -1 for logic level ‘0’
And hence two analog waves of different frequencies are obtained. The waveforms for BFSK are shown below.
Figure 17: A BFSK waveform derived from a Binary message.
3.3. Power Line Interface (Isolation Circuit): One of the most important parts of our power line transmitter and receiver is the Power Line Interface. Because our circuit has to connect to the 230V, 50 Hz power line, without careful isolation, the rest of the circuit will be easily damaged. The idea is to superimpose the data signal onto the 230V, 50 Hz power waveform, and extract it afterwards at the receiving end. The ideal isolation circuit should completely block the 50Hz signal, and pass the information signal. The information signal in our case is the frequency modulated signal.
3.4. Receiver: The signal generated by the transmitted is sufficiently amplified and passed through the isolation and coupling circuitry onto the A.C power lines. The high frequency digital signal gets superimposed over the low (50Hz) power lines. The signal thus passes over the power lines. This superimposed signal reaches the receiver. The receiver is designed to perform the following actions. Filtering and Isolation: The received signal is passed through a high pass (RC) filter which is designed specifically to filter out the 50 Hz power line signal. The powerful A.C signal should be prevented from entering the receiver peripherals as it can cause damage to the components. Thus the foremost function of the receiver must be to filter out this signal. Thereafter careful isolation from the 230V must be provided to the receiver components as it can also cause sufficient damage. At the output of this portion of the receiver circuit we get the transmitted high frequency digital signal.
Wave Shaping: The digital signal recovered from the transmitted signal can be affected by noise introduced during its travel through the power lines. Any stray noise pulses can cause unwanted action at the receiver. Therefore the receiver should wave shape the recovered digital signal to take care of the unwanted stray pulses.
26
Control Action: The recovered digital signal is then used by the microcontroller in the receiver to perform the necessary control action on the devices connected to the receiver. The receiver should thus differentiate between the different signals for the respective devices and the take the necessary controlling action.
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4. System Implementation: The Transmitter
28
4. System Implementation: The Transmitter
The chapters discussed earlier described the various design methodologies used in building a power line communication system. We now study the actual circuit of the transmitter and the receiver of the Power Line Communication system in chapter 4 & 5.
Let us start with the transmitter.
The basic layout of the transmitter circuit diagram is shown below.
29
Figure 18: The Transmitter Circuit diagram
4.1 Hardware:
•
The heart of the transmitter is the microcontroller. The system deploys the 8052 microcontroller because the timer2 in the 8052 can be programmed to generate a fixed frequency digital pulse train which can be used as a carrier in the amplitude shift keying modulation process. The microcontroller is programmed to output a frequency of 100 KHz at pin P1.0. The output from this pin is AND’ed with the signal from the transmit pin (TX) of the microcontroller. The output of the AND gate gives the amplitude modulated waveform. This waveform is subsequently fed to a transistor amplifier which amplifies the resultant waveform and also couples it to the input side of the opto-coupler circuitry.
•
•
30
•
The opto-coupler is used for the isolation of the microcontroller and other allied low voltage circuitry from the high voltage power line. The output of the optocoupler is capacitor coupled to the high voltage power line as shown in the figure.
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32
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4.2 Software
•
The transmitter microcontroller performs two main functions 1. Generating the carrier digital pulse train 2. Transmitting data bytes to control appliance at the receiver end.
•
The carrier frequency of 100 KHz is generated using the Timer2 of the 8052 microcontroller in the “Programmable Clock Out “mode. Here the timer2 is used to generate a continuous pulse train of 50% duty cycle having frequency of 100 KHz on pin P1.0.
•
We have programmed the microcontroller on the transmitter side to send four different bytes depending on the action to be performed.
•
Byte 1: Turns led 1 on Byte 2: Turns led 1 off
34
Byte 3: Turns led 2 on Byte 4: Turns led 2 off
•
These bytes are sent depending on the status of the pins 2.0, 2.1, 2.2, 2.3. The status of these pins is manipulated using four switches connected to these pins.
35
89C52 Programming for the Transmitter Circuit
include reg_52.pdf
cseg
at SJMP
00h MAIN 03H AJMP ZCD_INT 23h AJMP UART_INT
CSEG
AT
cseg
at
BSEG SEND_DATA_NOW: KEY_DTCT_1: KEY_DTCT_2: KEY_DTCT_3: KEY_DTCT_4:
AT DBIT DBIT DBIT DBIT DBIT
0 01 01 01 01 01
DSEG
AT
25H
DST_REP_COUNT:
DS
01
KEY1 KEY2 KEY3 KEY4
EQU EQU EQU EQU
P2.0 ; Clock pin assignment for I2C P2.1 ; Clock pin assignment for I2C P2.2 ; Clock pin assignment for I2C P2.3 ; Clock pin assignment for I2C
36
CSEG MAIN: MOV MOV MOV MOV MOV MOV SETB SETB SETB SETB SETB MOV MOV
AT
30H
T2CON,#04H T2MOD,#02H SCON,#50h TMOD,#20h TH1,#0f4h TL1,#0f4h TR1 ES EX0 IT0 EA RCAP2H,#0FFH RCAP2L,#0E2 ;2400 Baud-rate
HERE: JBC JB JB SETB MOV SEND_DATA_NOW,SEND_DATA KEY_DTCT_1,CHK_KEY2 KEY1,CHK_KEY2 KEY_DTCT_1 DST_REP_COUNT,#2
CHK_KEY2: JB KEY_DTCT_2,CHK_KEY3
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JB SETB MOV
KEY2,CHK_KEY3 KEY_DTCT_2 DST_REP_COUNT,#2
CHK_KEY3: JB JB SETB MOV KEY_DTCT_3,CHK_KEY4 KEY3,CHK_KEY4 KEY_DTCT_3 DST_REP_COUNT,#2
CHK_KEY4: JB JB SETB MOV SJMP KEY_DTCT_4,HERE KEY4,HERE KEY_DTCT_4 DST_REP_COUNT,#2 HERE
SEND_DATA: JB JB JB JB SJMP HERE KEY_DTCT_1,SEND_55 KEY_DTCT_2,SEND_AA KEY_DTCT_3,SEND_5A KEY_DTCT_4,SEND_A5
SEND_55: MOV SBUF,#55H
38
DJNZ CLR SJMP
DST_REP_COUNT,HERE KEY_DTCT_1 HERE
SEND_AA: MOV DJNZ CLR SJMP SBUF,#0AAH DST_REP_COUNT,HERE KEY_DTCT_2 HERE
SEND_5A: MOV DJNZ CLR SJMP SBUF,#5AH DST_REP_COUNT,HERE KEY_DTCT_3 HERE
SEND_A5: MOV DJNZ CLR SJMP SBUF,#0A5H DST_REP_COUNT,HERE KEY_DTCT_4 HERE
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; UART_INT:
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JBC JBC
TI,EXIT_UART RI,EXIT_UART
EXIT_UART: RETI
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ZCD_INT:
SETB SEND_DATA_NOW RETI END
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5. System Implementation: The Receiver
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5. System Implementation: The Receiver
We studied the transmitter in detail in the previous chapter. Now we shall look closer into the receiver of the power line communication system in this chapter. The signal received from the transmitter needs to be recovered and the necessary action is to be taken on the devices connected to the receiver.
The circuit layout of the receiver is given below.
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Figure 19: The Receiver circuit Diagram 5.1 Hardware:
•
The input at the receiving end is a 50 Hz power line signal superimposed with the 100 hz signal frequency. This composite signal is passed through a high pass RC filter to remove the 50 Hz component and pass the 100 KHz component(data signal). The signal is then amplified and fed to the input of the opto-coupler. The optocoupler isolates the low voltage microcontroller section from the high voltage section consisting of the power line. The output of the opto-coupler is again amplified and fed to the microcontroller where necessary processing takes place.
•
•
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5.2 Software
•
The receiver microcontroller performs two main functions 1. Wave shaping of the received signal to eliminate the effect of noise transients and for signal smoothening 2. Switching the devices connected to the pins based on the code received.
•
The received signal as a result is fed at the external interrupt 1. The interrupt subroutine performs the necessary wave shaping and gives the wave shaped output on pin P1.3. Pin P1.3 is then connected to the receive (RX) pin of the microcontroller. Depending on the byte received the microcontroller switched the LED’s connected to the pin P2.0 and pin P2.1.
•
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89C52 Programming for the Receiver Circuit
include reg_52.pdf
cseg
at SJMP
00h MAIN 13H SJMP EXT_1 0BH SJMP TMR_0 23h SJMP UART_INT
CSEG
AT
CSEG
AT
cseg
at
DSEG SR_DATA:
AT DS
25H 01
BSEG F_T_FLAG: VALID_DATA:
AT DBIT DBIT
00 01 01
OP1 OP2
EQU EQU
P2.0 P2.1
; Clock pin assignment for I2C ; Clock pin assignment for I2C
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CSEG
AT
30H
MAIN: CLR CLR CLR MOV MOV MOV MOV SETB SETB SETB SETB SETB SETB MOV SETB OP1 OP2 P1.3 SCON,#50h TMOD,#22h TH1,#0f4h TL1,#0f4h TR1 ES ET0 EX1 TR0 IT1 IP,#04H EA ;2400 Baud-rate
HERE: JBC SJMP VALID_DATA,PROC_SR_DATA HERE
PROC_SR_DATA: MOV CJNE A,SR_DATA A,#55H,CHK_4_AA
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SETB SJMP
OP1 HERE
CHK_4_AA: CJNE CLR SJMP A,#0AAH,CHK_4_5A OP1 HERE
CHK_4_5A: CJNE SETB SJMP A,#5AH,CHK_4_A5 OP2 HERE
CHK_4_A5: CJNE CLR SJMP A,#0A5H,HERE OP2 HERE
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; EXT_1: JB SETB SJMP F_T_FLAG,CONT_EXT_1 F_T_FLAG CONT_EXT_2
CONT_EXT_1: SETB P1.3
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CONT_EXT_2: MOV SETB RETI TL0,#220 TR0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; TMR_0: CLR CLR CLR RETI P1.3 F_T_FLAG TR0
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; UART_INT: JBC JBC TI,EXIT_UART RI,CHK_RCV_DATA
EXIT_UART: RETI
CHK_RCV_DATA:
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MOV CJNE RETI
A,SBUF A,#0,CHK_4_FF
CHK_4_FF: CJNE RETI A,#0FFH,GO_VALID_DATA
GO_VALID_DATA: MOV SETB RETI ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; SR_DATA,A VALID_DATA
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5.3 THE POWER LINE COMMUNICATION SYSTEM
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6.
Cost of Project
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Sr no: 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
Components PCB Microcontrollers 89C52 MCT2E PC817 Box caps BC548 LED LM2940 Capacitor 1000uF Capacitor 220uF Capacitor 0.1uF Capacitor 0.33pF Crystal Resistor Network Bridge rectifier Capacitor 10uF Bus ship Slide Switch Sockets AND gate Adapter Resistors Switches Transformers Diode Wires
Price(Rs) 50 40 7.5 3 7 6 1 35 5 3 .25 .25 3 3 5 2 4 3 6 8 100 1 4 200 .25 1
Quantity 2 2 2 3 3 1 2 2 2 2 3 4 2 3 2 2 2 2 3 1 3 10 4 1 1 20
Total 100 80 15 9 21 6 2 70 10 6 .75 1 6 9 10 4 8 6 18 8 300 10 16 200 .25 20
Grand Total
Rs.918
7. Conclusion
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The devices connected to the designed power line communication system can be increased by making minute changes in the underlying hardware and software. The system can be used for a number of applications such as home automation, automatic meter reading etc. The implications of power line communication are farfetched and varied. The technology can be used for controlling appliances in a household as well as for remotely controlling vast industrial establishments. The technology is highly cost effective since it uses the already established power line network for data communications. In spite of its numerous advantages, the communication is limited to low data rates and hampered by high power line noise. Research advances in these areas will further lead to improvements in the technology.
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8. Bibliography
www.hometoys.com www.lammertbies.nl. Design of Power-Line Communication System (PLC) Using a PIC Microcontroller, Q. Al-Zobi, I. Al-Tawil, K. Gharaibeh and I. S. Al-Kofahi. ITU Press Release: New global standard for fully networked home. IEEE P1901 Press Release. Modicon MODBUS Protocol Reference Guide. Tech note-X-10 Communications Protocol and Power Line Interface.
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9. APPENDIX
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