Schweitzer Engineering Laboratories SEL-749M, Instruction Manual

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D.4
SEL-749M Relay Instruction Manual Date Code 20080918
Modbus RTU Communications Protocol
Communications Protocol
The relay responses to errors in the query are shown in Table D.7 .
06h Preset Single
Register Command
The SEL-749M uses this function to allow a Modbus master to write directly
to a database register. Refer to the Modbus Register Map in
Table D.23 for a
list of registers that can be written by using this function code. If you are
accustomed to 4X references with this function code, for six-digit addressing,
add 400001 to the standard database addresses.
In Table D.8 , the command response is identical to the command required by
the master.
The relay responses to errors in the query are shown in Table D.9 .
Table D.6 04h Read Input Register Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (04h)
2 bytes Starting Register Address
2 bytes Number of Registers to Read
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (04h)
1 byte Bytes of data ( n )
n bytes Data (2–250)
2 bytes CRC-16
Table D.7 Responses to 04h Read Input Register Query Errors
Error Error Code Returned
Communication
Counter Increments
Illegal register to read Illegal Data Address (02h) Invalid Address
Illegal number of registers to read Illegal Data Value (03h) Illegal Register
Format error Illegal Data Value (03h) Bad Packet Format
Table D.8 06h Preset Single Register Command
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (06h)
2 bytes Register Address
2 bytes Data
2 bytes CRC-16
Table D.9 Responses to 06h Preset Single Register Query Errors
Error Error Code Returned
Communication Counter
Increments
Illegal register address Illegal Data Address (02h) Invalid Address Illegal Write
Illegal register value Illegal Data Value (03h) Illegal Write
Format error Illegal Data Value (03h) Bad Packet Format
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