Sel 551 - Table G.6 03 H Read Holding Register Command; Table G.7 04 H Read Holding Register Command

G.5
Date Code 20110408 Instruction Manual SEL-551 Relay
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
The relay responses to errors in the query are shown below:
04h Read Input
Registers Command
Use function code 04h to read from the Modbus Register map shown in
Table G.18. You may read a maximum of 125 registers at once with this
function code.
Table G.6 03h Read Holding Register Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (03h)
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 (03h)
1 byte Number of Bytes of Data (n)
n bytes Data
2 bytes CRC-16
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 G.7 04h Read Holding 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 Number of Bytes of Data (n)
n bytes Data
2 bytes CRC-16