MODBUS Serial Communication Protocol
Contents
1. a eee enema 8 1 ICH 63 O MODBUS 2 20 References Cross references Icons for emphasis 2 HOWTO USE THIS MANUAL This manual is designed to permit quick easy reference with the following features References column A column to the left of the text contains references to subjects discussed in the text to help you locate the information you need quickly and easily Cross references All words written in italics are referenced in the subject index to help you find the page containing details on this subject supposing you read the following text when the alarm is triggered the compressors will be shut down The italics mean that you will find a reference to the page on the topic of compressors listed under the item compressors in the index If you are consulting the manual on line using a computer words which appear in italics are hyperlinks just click on a word in italics with the mouse to go directly to the part of the manual that discusses this topic Some segments of text are marked by icons appearing in the references column with the meanings specified below2. a Take note marks a specific note on the topic that the user should bear in mind Tip marks a tip that can help the user understand and use the information on the topic better Warning marks information that if not thoroughly comprehended may badly affect the system or be hazardous for people equipment data etc users must read these warnings MODBUS 3 20 eS broadcast a 3 INTRODUCTION The Modbus protocol is for communications between devices connected together over a network Network The system is based on the master slave mechanism where only the master device can initiate communications The other devices slaves respond either by supplying data or performing the operations requested by the master gt 5 Generally speaking the protocol describes the routines a device has to use to request access to another device respond to queries from other controllers or devices detect and signal errors and lastly establish a common format for both content and layout in the various fields of the message 3 1 Modbus Network As regards the physical connection the devices normally use RS232 compatible interfaces for point to point communications or RS485 for multidrop connections The type of connection wiring signal levels and communications parameters such as parity and baud rate are defined Controllers can be connected directly over a network or via modem Each de
3. MODBUS Serial Communication Protocol User Manual 1 CONTENTS Vo E E 2 2 HOW 0 use this MANUAL ou eceeeeeeceececececsccceccssccesscessceeccceccecccecocscsceccecsccesscessccesccessceesceescecessenseensecsacesscensceesees 3 3 1 Modbus NETWORK anne 3 2 Other types of network 4 The Modbus standard protocol 4 1 NTE 4 2 query response Serial transmission 4 3 1 Character serial transmission 44 SPCC Wib RTO 7 4 4 1 ETE N 7 4 4 2 Function code 4 4 3 Data bytes 4 4 4 ARE E Pirate Heer eier 5 ATO CONO G 5 1 PRON E 9 52 5 2 1 CRC calculation algorithm 5 3 2 cbs aan EE E A EE ne S E So a ue Ne PS AOE 7 E E ee ne oe dean see nad ag ceed a eee 7 1 Funcuons and EMITS
4. The interval is set sufficiently long so that all slave devices can respond normally If the slave detects a transmission error the message will not be taken into consideration and the slave not create a response for the master In this way the timeout will end and will permit the master program to handle the error MODBUS 9 20 For example for the Energy 400 device the maximum length of time that can elapse between sending a packet from the master and the slave s response is 4 ms A message addressed to a non existent slave device will not create a timeout either Note Other networks such as MAP or Modbus Plus use a frame check at a higher level than the Modbus content of the message On these networks the CRC check field of the Modbus message is not applied In the event of a transmission error the specific protocol of these networks will notify the originating device of the message the master since the slave will make no attempts whatsoever that an error has occurred and will permit it to make another attempt or to cancel the send depending on how it is set If the message has been delivered but the slave device cannot respond there is an error similar to a timeout that can be detected by the master s program MODBUS 10 20 Function code field Data field 6 DIAGNOSTICS Except for messages such as broadcasts when a master device sends a query to a slave it waits for a response Any of the following 4 cases may
5. 38 01 0011 1000 0000 0001 Hexadecimal value to indicate the logic area 7 and the index 1 of the resource vent 1 00 01 0000 0000 0000 0001 Hexadecimal value to indicate the number of variables to write 02 0000 0010 No of data bytes 00 0000 0000 If the area is a set of bytes this field is always on zero XX Value to write The hexadecimal value 38 01 has been obtained by applying the data field format for the logic area Taking the first 5 bits 00111 we get the decimal value 7 while the remaining bits give us the logic address of the variable to write in this case it is 1 The following 2 bytes 00 01 indicate the number of variables to write after the one in the previous address in the example at issue there is just one variable to write Response ee 2322 8 bit binary Description 01 0000 0001 Address of the slave in the network 10 0000 0011 Function code echo 38 01 0000 0010 Hexadecimal value to indicate the logic area 7 and the index 1 of the resource vent 1 MODBUS 15 20 00 01 data written MODBUS 16 20 ASCII RTU 8 APPENDIX 8 1 ASCII coding When the controllers set up for working in a Modbus network that uses ASCII coding American Standard Code for Information Interchange each byte eight bits 2 hexadecimal characters represents an alphanumeric character The greatest advantage of this coding i
6. CRC NISRA NIN The Byte count field indicates the number of bytes 8 bits contained in the Data field The Force data field specifies the values to enter in the variables indicated Example writing the value ON 01 hex in the digital output RL1 on the slave with address 1 The resource is indexed its logic address on 1 Query 01 OF 28 01 00 01 01 01 Response 01 OF 28 01 00 01 MODBUS 14 20 Function code 16 10 hex eS Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network OF 0000 1111 Function code 2801 0010 1000 0000 0001 Hexadecimal value to indicate the logic area 5 the index 1 of the resource RL1 00 01 0000 0000 0000 0001 Hexadecimal value to indicate the number of variables to write 01 0000 0001 No of data bytes 01 0000 0001 Value ON The hexadecimal value 28 01 has been obtained by applying the data field format for the logic area Taking the first 5 bits 00101 we get the decimal value 5 while the remaining bits give us the logic address of the variable to write in this case it is 1 The following 2 bytes 00 01 indicate the number of variables to write after the one in the previous address in the example at issue there is just one variable to write In the Force data field the values to write in the variables are grouped 8 bits at a time and are represented by a hexadecim
7. bits 2 stop bits if no parity 1 011000011 the ASCII code for 6 11 8 3 ASCII framing If you use ASCII coding the frames messages start with a colon ASCII hex and end with carriage return line feed or CRLF ASCII OD hex and hex In transmitting all the other fields of the message the hexadecimal characters used 0 9 A F The devices connected together the network are constantly analysing the network bus waiting for the colon When they receive this character each device decodes the next field address field to find out whether its address is the one specified There can be time intervals of up to one second between the characters in the message If there is a longer time interval between characters the receiving device assumes that some kind of error has occurred during the transmission MODBUS 17 20 Logical OR Exclusive OR Stand by Reset Range Master Slave Baud rate LSb MSb BUS Timeout interval Nibble 9 GLOSSARY Multiple inputs with an OR relationship to one another are equivalent to a single input with the following status Active if at least one input is active Inactive if no input is active Two inputs related to one another are equivalent to a single input with the following status Inactive if both inputs are active or both are inactive Active if only one of the two is active Indicates that the instrument is wait
8. Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 04 0000 0011 Function code 10 01 0001 0000 0000 0001 Hexadecimal value to indicate the logic area 7 and the index 1 of the resource 1 00 01 0000 0000 0000 0001 Hexadecimal value to indicate the number of consecutive variables to read The hexadecimal value 10 01 has been obtained by applying the data field format for the logic area Taking the first 5 bits 00010 we get the decimal value 2 while the remaining bits give us the logic address of the variable to read in this case it is 1 The following 2 bytes 00 01 specify the number of variables to read after the one contained in the previous address in the example at issue there is just one variable to read 8 bit binary Description 01 0000 0001 Address of the slave in the network 04 0000 0011 Function code echo 02 0000 0010 No of data bytes XX XXXX XXXX Values contained the variable to read XX XXXX XXXX Values contained the variable to read The function code 15 OF hex is used for writing the digital variables contained in the resources of the slave which have digital inputs logic area 2 dec The broadcast service is implemented with this function Field No of bytes Slave address Function code First bit address Number of bits Byte count Force data
9. R code received in the query is not a possible action for the The data address in the data field is not a valid address for the slave 02 Illegal data address Area not corresponding to the function index 0 index non existent in the requested area 03 data addres The figure in the data field is not accessible for the slave Too many data 5 requested for the area content tx buffer exceeded in the response ADD COM CRC 01 81 02 CRC CRC RESPONSE WITH ERROR 02 TO FUNCTION 01 MODBUS 11 20 a Function code 01 01 hex 7 FUNCTIONS This section provides some examples on using the functions implemented in Eliwell Microtech devices The following table sums up the commands available in the various logic areas Function Logic area code decimal Area Description Function description Type of action of the function decimal 1 5 Digital outputs Reading digital variable Obtains the values of one or more 2 4 Digital inputs digital variables 1 Parameters 6 7 Analogue outputs Reading analogue variables Obtains the values of one or more analogue variables 8 RAM 4 2 Analogue inputs i aes Forces the value of one 15 5 Digital outputs Writing digital variables digital variables 1 Parameters 6 EEPROM Forces the value of or more 16 Writing analogue variables i 7 Analogue outputs analogue va
10. al value For example if we have values type ON ON OFF OFF ON ON OFF ON they correspond to the binary value 1100 1101 that is the hexadecimal value CD hex er RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network OF 0000 0011 Function code echo 28 01 0000 0010 Hexadecimal value to indicate the logic area 5 and the index 1 of the resource RL1 00 01 No data written corresponds to the no of data requested The function code 16 10 hex is used to write the analogue variables contained in the resources of the slave The logic areas to which the function refers are Parameters EEPROM Analogue outputs RAM The broadcast service is implemented with this function Field No of bytes Slave address Function code First bit address Number of bits Byte count Force data CRC nN sSs i nini The Byte count field indicates the number of bytes 8 bits contained in the Data field The Force data field specifies the values to enter in the variables indicated Example writing the value xx in the analogue output vent 1 on the slave with address 1 The resource vent 1 is indexed its logic address on 1 Query 01 10 38 01 00 01 02 00 xx Response 01 10 38 01 00 01 Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 10 0000 1111 Function code
11. alculation of the CRC function Cyclical Redundancy Check that is performed on the entire message The CRC field is added to the message as the last field When the CRC field is formed the low order byte is set first followed by the high order byte The high order byte of the CRC is the last byte in the message to be sent MODBUS 8 20 parity 5 ERROR CONTROLS The standard Modbus serial network uses three types of error check e parity check odd even that be applied as an option on each character forming the frame e frame check CRC on the other hand is applied to the entire message Timeout 5 1 Parity check Users can configure devices to perform the Even or Odd Parity check or for a No Parity check This will determine how the parity bit is set for each character If Odd or Even Parity has been specified the number of bits ON 1 will be counted in the data section of each character seven data bits for ASCII coding eight for RTU coding The parity bit will then be set to 0 or 1 if the total number of bits equal to 1 is even or odd For instance supposing that these eight bits form a character transmitted with RTU coding 1100 0101 The total number of bits in the frame equal to 1 is four If Even Parity has been set the frame parity will be equal to 0 since the total number of bits equal to 1 including the parity bit is four which is an even number Whereas if we use Odd P
12. arity the parity bit will be set to 1 thereby making the total an odd number there are 5 bits equal to 1 When the message is transmitted the parity bit is calculated and applied to each character in the frame The receiving device counts the number of bits equal to 1 and returns an error if this is not the same as the parity setting all devices in a Modbus network have to be configured to have the same parity check method Notice that the parity check can only detect an error if there is an odd number of bits in error If for instance we have set the Odd Parity check and two 1 bits have been lost from a character containing three 1 bits the result will still be an odd number of bits equal to 1 Lastly if we have specified No Parity no check is made and a stop bit is added instead of the parity bit 5 2 CRC check In the RTU coding the messages include an error check field that is based on the CRC method The CRC field checks all the content of the entire message This check is applied irrespective of any parity method being used to verify the single characters of the message The CRC field comprises two bytes containing a binary value of 16 bits The CRC value is calculated by the transmitting device which adds it to the message The receiving device when reading the message recalculates the CRC and compares it with the value received in the CRC field If these two values not the same there is an error 5 2 1 CRC calcula
13. er serial transmission sans 7 M E E 9 1 18 CRC Calculation GIGOLIth Messen 9 Modbus 4 2 1 29599 9 ER EER 18 CRC field 8 Creating frames with RTU Coding 7 18 2 2222000 3 r E E PE 5 P PO I E 9 QUEI 6 R daaaniieeita 18 5 222200 0 3 BASED 18 Res pone 6 17 7 5 Serial transmission 6 SITE 18 DUGG UY ahaa Sin E I 18 T THE MODBUS STANDARD 1 6 query response 6 9 Timeout 18 ei MODBUS 2000 09 Cod 8MA10016
14. ing in stand by mode all functions are suspended Means set to zero Values falling within a given interval Range 1 100 indicates all values between 1 and 100 In communications between two devices the term Master specifies the device that starts governs and ends the communication In communications between two devices the term Slave specifies the device that responds to and obeys the requests of the Master The baud rate Baud measures the transmission speed of a channel 1Baud rate 1 bit 1s Least significant bit Most significant bit Type of physical connection between devices All the devices are connected to the same physical line Timeout is the time limit on waiting to receive a message after which an error is considered to have occurred A nibble is a set of 4 bits e g 1001 MODBUS 18 20 MODBUS 19 20 10 ANALITIC INDEX G 18 APPENDIK nunmal 17 HOW TO USE THIS 3 Bw 17 BIN 17 Icons for 3 nun Rasensitaale 4 18 L AEE 4 Logic 8 18 Logical OR 18 18 Charact
15. n code with the most significant bit MSb set to one logic 1 For example a message from the master to the slave to read a set of registers would have the following function code 0000 0011 hexadecimal 03 If the slave receives the query and performs the envisaged action without error then it will return the same code in its response If there is an exception the slave will return 1000 0011 hexadecimal 83 MODBUS 7 20 Data field Logic areas The slave will put a single code in the data field describing the type of error that occurred or the reason for the exception The application program of the master device is responsible for handling the exception responses Typical actions are successive attempts at resending the message sending a message to the slave or notifying the operators 4 4 3 Data bytes The data field is created using blocks of two hexadecimal figures 1 byte in the range between 00 and FF These form a single RTU character of eight bits The data field of the message sent by the master to the slave device contains the additional information to use to perform the action defined in the function code In particular in Eliwell Microtech devices the first two bytes are basically used to gain access to the resources of the slaves Each resource has a logic address where it be seen to belong to an area and to offset inside the area according to the following outline Area Offset addres
16. number of consecutive variables to read The hexadecimal value 38 01 has been obtained by applying the data field format for the logic area Taking the first 5 bits 00111 we have the decimal value 7 while the remaining bits give us the logic address of the variable to read in this case it is 1 The following 2 bytes 00 01 specify the number of variables to read after the one contained in the previous address in the example at issue there is just one variable to read MODBUS 13 20 Function code 04 04 hex Function code 15 OF hex oe 8 bit binary Description 01 0000 0001 Address of the slave in the network 03 0000 0011 Function code echo 02 0000 0010 No of data bytes 00 0000 0000 If the area is a set of bytes this field is always on zero XX XXXX XXXX Values contained the variable to read The function code 04 04 hex is used to read the analogue variables contained in the resources of the slave which have analogue inputs logic area 2 dec The broadcast service is not implemented with this function Field No of bytes Slave address Function code First word address Number of words CRC NIN IN Example reading the analogue input ST1 on the slave with address 1 The resource ST1 is indexed it logic address on 1 Query 01 04 10 01 00 01 Response 01 04 02 xx xx
17. occur e If the slave receives the message without any errors and it can handle the queries normally it returns a normal response If the slave fails to receive the query due to communications error it gives no response If the slave receives the query but it detects communications error parity it gives no response The master program will evaluate whether there is a timeout condition If the slave receives the query without any communications errors but it is not able to handle it for instance if the query is to read a register that does not exist the slave will give an exception response informing the master of the nature of the error The exception response has two fields that distinguish it from a normal response the function code field and the data field In a normal response the slave repeats the master s query function code All function codes have 0 as their most significant bit MSb since their hexadecimal values are always lower than 80 In an exception response the slave sets the most significant bit MSb of the function code to 1 The master program may acknowledge the exception response and then examine the data field to find the error code In a normal response the slave returns the required information In an exception response the slave returns the error code as specified in the following table Code Name Meaning 01 Illegal function T
18. ode 0 00083 4 4 1 Device Address The address field of a message contains eight bits RTU Valid addresses for slave devices lie in the range between 0 and 255 decimal values A master device addresses the message to a designated slave by putting the slave s address in the device address field of the message When a slave responds it puts its own address in the address field so the master can identify it The address 0 is used for the Broadcast address which identifies all the slaves Slaves do not respond to a broadcast query In some Microtech Eliwell instruments the address field is interpreted as two nibbles respectively specifying the family and the device address within the family example The address 11010011 is interpreted as 1101 Family 11 0011 Device 3 When the Modbus protocol is used a higher level network other types of network e g on TCP IP the broadcast service might not be permitted or it could be replaced by other methods The devices connected to the network constantly analyse the network bus also during periods of silence When a device receives the first field address field it decodes it to find out whether the address it contains is its own in which case it goes on to read the rest of the message In sequence straight after the last character transmitted an interval of at least 3 5 character time marks the end of the message after which a new message can commence The entire f
19. rame of the message has to be transmitted in a continuous stream If there is an interval longer than 1 5 character time before completing the frame the receiving device will delete the incomplete message and consider the next byte received as the start of the address field of a fresh message Likewise if a new message starts sooner than the interval of 3 5 character time following a previous message the receiving device will consider it as a continuation of the former one This would cause an error in the final value of the CRC which would establish that joining the two messages is not legitimate 4 4 2 Function code The function code field of a message contains eight bits RTU Valid codes lie in the range from 1 to 255 decimal values Only a few functions of the modbus protocol are implemented in the Eliwell Microtech controller When a message is sent from a master to a slave device the function code field specifies the kind of action to take For example read the ON OFF status of a set of digital variables read the data of a set of registers write to a specific register When the slave responds to the master it uses the function field to indicate either a normal response if there are no errors or some kind of error that has occurred exception response For a normal response the slave simply repeats echo the original code In the case of an exception response the slave returns a code that is equivalent to the original functio
20. riables 8 RAM 7 1 Functions and Examples The examples refer to the function and area they hold for any instrument implementing them The function code 01 01 hex is used to read digital variables contained in the slave resources which have digital outputs logic area dec The broadcast service is not implemented with this function Field No of bytes Slave address Function code First bit address Number of bits CRC NININI l Example reading the digital output RL1 on the slave with address 1 The resource RL1 is indexed that is it has its logic address on 1 Query 01 01 28 01 00 01 Response 01 01 01 xx Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 01 0000 0001 Function code 28 01 0010 1000 0000 0001 Hexadecimal value to indicate the logic area 5 and the index 1 of the resource RL1 00 01 0000 0000 0000 0001 Hexadecimal value to indicate the consecutive number of variables to read The hexadecimal value 28 01 has been obtained by applying the data field format for the logic area Taking the first 5 bits 00101 we get the decimal value 5 while the remaining 11 bits give us the logic address of the variable to read in this case it is 1 The following 2 bytes 00 01 specify the number of variables to read after the one contained in the previous address in the example at issue there i
21. rs of the Modbus network via the relevant software applications and drivers will make the conversion between the Modbus protocol and the specific protocol used by the network MODBUS 5 20 Frames Query 5 4 THE MODBUS STANDARD PROTOCOL This section examines the technical and operating specifications of the Modbus protocol in greater detail 4 1 Frames Communication between two devices via the modbus protocol takes place with an exchange of frames structure In a frame composed of a set of bits there is a start an end and an internal structure that can be exemplified by the following outline Device Function 8 bits Data Byte Error Check The maximum length of a packet is closely tied to the transmission reception buffer of the interface used It is therefore a function of the resources In any case a message can be at most 256 bytes long 4 2 The query response cycle A communications cycle between a master and a slave device takes place by an exchange of frames QUERY Master Device Device Function Function if Eight bit Data Bytes Eight bit Data Bytes Error 5 5 query will contain the following fields Device address address of the slave the network This address necessarily needs to be the same both the query and the response Function code The function code in the quer
22. s 5 most significant bits MSb to identify the area gt 32 possible areas 11 least significant bits 150 to identify the offset gt 2048 possible addresses inside each area The areas be divided up into parameters analogue inputs timer Each area can be accessed for reading or writing with the function code specified here area description Reading function Writing function function code function code 1 parameters 3 16 2 analogue inputs 4 3 timer 3 16 4 digital inputs 2 5 digital outputs 1 15 6 EEPROM 3 16 7 analogue outputs 3 16 8 RAM 3 16 That is functions function function description action 1 2 reading digital variable obtains the value of one or more digital variables 3 4 reading analogue variable obtains the value of one or more analogue variables 15 writing digital variables forces the value of one or more digital variables 16 writing analogue variables forces the value of one or more analogue variables Areas 6 and 8 are used when the limited availability of device resources does not permit using logic areas Then the specified address is the real physical address of the resource in RAM or in EEPROM 4 4 4 CRC field When using RTU transmission coding to form message frames the error check field contains a 16 bit value composed of two bytes of eight bits each The error check characters are the result of the c
23. s 00100 we have the decimal value 4 while the remaining bits give us the logic address of the variable to read in this case it is 2 The next 2 bytes 00 02 specify the number of variables to read after the one contained in the previous address in the example at issue there are two variables to read hs RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 02 0000 0010 Function code echo 01 0000 0001 No of data bytes XX XXXX XXXX Values contained the variables to read The function code 03 03 hex is used to read the analogue variables contained in the resources of the slave The logic areas to which the function refers are Parameters EEPROM Analogue outputs RAM The broadcast service is not implemented with this function Field No of bytes Slave address Function code First word address Number of words CRC NININ Example reading the vent output 1 on the slave with address 1 resource vent1 is indexed its logic address 1 Query 01 03 38 01 00 01 Response 01 03 02 00 xx Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 03 0000 0011 Function code 38 01 0011 1000 0000 0001 Hexadecimal value to indicate the logic area 7 and the index 1 of the resource vent 1 00 01 0000 0000 0000 0001 Hexadecimal value to indicate the
24. s just one variable to read an RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 01 0000 0001 Function code echo 01 0000 0001 Hexadecimal value to indicate the index 1 of the resource RL1 XX XXXX XXXX Values contained the variables to read MODBUS 12 20 Function code 02 02 hex Function code 03 03 hex The function code 02 02 hex is used to read the digital variables contained in the resources of the slave which have digital inputs logic area 4 dec The broadcast service is not implemented with this function Field No of bytes Slave address Function code First bit address Number of bits CRC NINN Example reading the digital inputs ID2 and ID3 on the slave with address 1 The resources ID2 and ID3 are indexed their logic address on 2 and 3 respectively Query 01 02 20 02 00 02 Response 01 02 01 xx Query hex RTU 8 bit binary Description 01 0000 0001 Address of the slave in the network 02 0000 0010 Function code 20 02 0010 0000 0000 0010 Hexadecimal value to indicate the logic area 4 and the index 2 of the resource ID2 00 02 0000 0000 0000 0010 Hexadecimal value to indicate the number of consecutive variables to read The hexadecimal value 20 01 has been obtained by applying the data field format for the logic area Taking the first 5 bit
25. s that it allows a time interval between two characters of up to one second 1 s without an error being signalled Each character is sent with 10 bits used as described here start bit 7 data bits LSb least significant bit sent first bit for odd even parity no bit for no parity stop bit if parity is used or 2 stop bits for no parity For example coding the decimal figure 63 requires 2 characters 6 and 3 and therefore 2 transmissions start bit 7 data bits 2 stop bits if no parity 1 0110110 the ASCII code for 6 11 1 0110011 the ASCII code for 3 11 8 2 RTU coding When the controllers are set up for communicating over a Modbus network that uses RTU coding Remote Terminal Unit each byte eight bits represents two hexadecimal characters 4 bits each The greatest advantage in using this coding consists of the higher density of characters it permits more efficient faster transmissions than ASCII for the same baud rate Each character is sent with 10 bits used as described here start bit 8 data bits LSB least significant bit sent first bit for odd even parity no bit for no parity 1 stop bit if parity is used 2 stop bits for no parity For example coding the decimal figure 63 requires 2 hexadecimal characters 6 and 3 that converted into binary format are 0110 6 and 0011 3 They are joined together to form a single byte so there is just one transmission start bit 8 data
26. the same for all the devices on the network The coding used by the Eliwell Microtech devices is RTU in the remainder of this manual we will therefore always either explicitly or implicitly refer to this type of coding 4 3 1 Character serial transmission When messages are transmitted over a Modbus standard serial network each character or byte is sent in this order from left to right Least Significant Bit LSD Most Significant Bit MSb MODBUS 6 20 framing Function code a With the RTU characters the sequence is as follows With parity check byte Start LSb MSb Parity Stop Without parity check byte Start LSb MSb Stop Stop 4 4 Creating frames with RTU coding A frame created with RTU coding has the following structure Start Device Address Function Code Data Bytes ErrorCheck CRC End 1 2 3 4 8 bit 8 bit nx 8 bit 16 bit 1 2 3 4 RTU coding messages start with a silence lasting at least equal to the time for transmitting 3 5 characters Character time It is more common practice to use a multiple of the transmission time of a character shown as 1 2 3 4 in the figure Where T nth transmission time of 1 character If the transmission speed is set to 9600 baud rate the transmission time of one character will be equal to 1 9600 8 number of bits to make one character in RTU c
27. tion algorithm Passo n 1 Set a 16 bit variable with the hex value FFFF equal to all 1 in the binary system This variable is called the CRC register Passo n 2 Compare the first byte of eight bits of the message with the low order byte of the CRC register using the exclusive OR operator Put the result in the CRC register Passo n 3 Shift the CRC register one bit to the right towards the LSB filling the MSB with 0 Take out the content of the LSB and examine it Passo n 4 If the value of the LSB is zero repeat Step no 3 If the LSB is 1 compare the CRC register using the exclusive OR operator with the hex polynomial value A001 1010 0000 0000 0001 Passo n 5 Repeat Step no 3 and no 4 until all eight shifts have been performed Then another byte will be processed Passo n 6 Repeat Steps from no 2 to no 5 for the following bytes of the message Continue until all the bytes have been processed Result The content of the CRC register is the required CRC value Passo n 7 When the CRC is put in the message the higher and lower bytes need to have their positions swapped over For example if the CRC value is hex 1241 0001 0010 0100 0001 the message will be Function Address de Data count Data Data Data Data CRC Lo 41 CRC Hi 12 5 3 Timeout The master is generally configured by the user to wait for a set timeout interval before cancelling the communication transaction
28. vice must be assigned a unique address for the entire network During communications over a Modbus network the protocol establishes how each device recognises a message addressed to it determines the type of action to take extracts the data and any other information contained in the message If a response is envisaged the slave device using the Modbus protocol will in its turn create and send the message The protocol makes it possible to send messages either to a single slave specifying the address in the query packet or to all the slaves on the network via a broadcast message The slave responds to the master s queries only if individually called upon addressed Slaves do not respond to broadcast messages broadcast no response system In particular the queries of the master device will comprise Device address broadcast address Function code that defines the required action e A Set of data e error check field Similarly the responses of a slave device will contain the fields that confirm the actions performed the data that need to be returned and an error check field If an error occurs in communicating the message or if the slave cannot perform the requested action the slave will create an error message and send it in response 3 2 Other types of network On other types of network the messages containing the Modbus protocol are enclosed in the frames of the network used MODBUS 4 20 The controlle
29. y tells the slave device concerned addressed the type of action to perform e Data bytes contain any additional information that the slave needs to perform the function Error check check error field provides method to check the integrity of the message content For example the function with code 03 decimal requests the slave to read the registers holding register and to respond with their values The data field in this example will contain the data informing the slave from which register variable to begin and for how many registers to continue reading If the slave creates a normal response the function code in the response is an echo of the query function code The data bytes field contains the data ollected by the slave such as the value of a register If an error occurs the function code is modified to indicate that the response is an error response and the data bytes field contains a code describing thle error The error check field enables whether the message content is valid 4 3 Serial transmissidn mode The instruments can be configured to communicate with the Modbus network standard using two transmission codes ASCII and RTU They establish how the bits form the fields of the frame The type of coding used is pre set by the firm or can be selected together with the communications parameters parity baud rate etc The type of coding ASCII RTU and relevant parameters parity baud rate must be
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