Schweitzer Engineering Laboratories SEL-751 - Table E.10 Responses to 03 H Read Holding Register Query Errors; Table E.8 Responses to 02 H Read Input Query Errors; Table E.9 03 H Read Holding Register Command

E.6
SEL-751 Relay Instruction Manual Date Code 20170927
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown in Tab le E.8.
03h Read Holding
Register Command
Use function code 03h to read directly from the Modbus Register Map shown
in Table E .3 4. 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 Tab le E.10.
04h Read Input
Register Command
Use function code 04h to read directly from the Modbus Register Map shown
in Table E .3 4. You can read a maximum of 125 registers at once with this
function code. Most masters use 3X references with this function code. If you
are accustomed to 3X references with this function code, for five-digit
addressing, add 30001 to the standard database address.
Table E.8 Responses to 02h Read Input Query Errors
Error Error Code Returned
Communications
Counter Increments
Invalid bit to read Illegal Data Address (02h) Invalid Address
Invalid number of bits to read Illegal Data Value (03h) Illegal Register
Format error Illegal Data Value (03h) Bad Packet Format
Table E.9 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 E.10 Responses to 03h Read Holding Register Query Errors
Error Error Code Returned
Communications
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