E.8
SEL-787 Relay
Instruction Manual
Date Code 20081022
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown in
.
03h Read Holding
Register Command
Use function code 03h to read directly from the Modbus Register Map shown
in
. 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
04h Read Input
Register Command
Use function code 04h to read directly from the Modbus Register Map shown
in
. 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 E.8
Responses to 02h Read Input Query Errors
Error
Error Code Returned
Communication
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
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
Summary of Contents for SEL-787
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