E.15
Date Code 20170927 Instruction Manual SEL-751 Relay
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are as follows.
62h Read
Enumeration
Text Command
The SEL-751 uses this function to allow a Modbus master to read parameter
enumeration or bit enumeration values (setting lists) from the relay. One
parameter enumeration is read in each query.
The relay responses to errors in the query are as follows.
7Dh Encapsulated
Packet With Control
Command
The SEL-751 uses this function to allow a Modbus master to perform control
operations and another Modbus function with one query. The Device Net card
transmits this command periodically to achieve high-speed I/O processing and
establish a heartbeat between the DeviceNet card and the main board.
Table E.27 61h Read Parameter Text Query Error Messages
Error Error Code Returned
Communications
Counter Increments
Illegal parameter to read Illegal Address (02h) Invalid Address
Table E.28 62h Read Enumeration Text Command
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (62h)
2 bytes Parameter Number
1 byte Enumeration Index
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (62h)
2 bytes Parameter Number
1 byte Enumeration Index
16 bytes Enumeration Text
2 bytes CRC-16
Table E.29 61h Read Parameter Enumeration Text Query Error Messages
Error Error Code Returned
Communications
Counter Increments
Illegal parameter to read Illegal Address (02h) Invalid Address
Illegal enumeration in index Illegal Data Value (03h) Illegal Register
Table E.30 7Dh Encapsulated Packet With Control Command (Sheet 1 of 2)
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Control Command (same as write to 2000h)
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