Schweitzer Engineering Laboratories SEL-749M - Table D.4 03 H Read Holding Register Command; Table D.5 Responses to 03 H Read Holding Register Query Errors

D.3
Date Code 20080918 Instruction Manual SEL-749M Relay
Modbus RTU Communications Protocol
Communications Protocol
Cyclical
Redundancy Check
The SEL-749M calculates a 2-byte CRC value through use of the device
address, function code, and data region. It appends this value to the end of
every Modbus response. When the master device receives the response, it
recalculates the CRC. If the calculated CRC matches the CRC sent by the
SEL-749M, the master device uses the data received. If there is no match, the
check fails and the message is ignored. The devices use a similar process
when the master sends queries.
03h Read Holding
Register Command
Use function code 03h to read directly from the Modbus Register Map shown
in Table D.23.
You can read a maximum of 125 registers at once with this function code.
Most masters use 4X references with this function code. If you are
accustomed to 4X references with this function code, for five-digit addressing,
add 40001 to the standard database address.
The relay responses to errors in the query are shown in Table D.5.
04h Read Input
Register Command
Use function code 04h to read directly from the Modbus Register Map shown
in Table D.23.
You can read a maximum of 125 registers at once with this function code.
Most masters use 4X references with this function code. If you are
accustomed to 4X references with this function code, for five-digit addressing,
add 40001 to the standard database address.
Table D.4 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 Bytes of data (n)
n bytes Data (2–250)
2 bytes CRC-16
Table D.5 Responses to 03h Read Holding 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
Courtesy of NationalSwitchgear.com