E.18
SEL-751A Relay
Instruction Manual
Date Code 20100129
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown below.
7Dh Encapsulated
Packet With Control
Command
The SEL-751A uses this function to allow a Modbus master to perform
control operations and another Modbus function with one query. The Device
Net card will transmit this command periodically to achieve high-speed I/O
processing and establish a heartbeat between the DeviceNet card and the main
board.
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.27
61h Read Parameter Enumeration Text Query Error Messages
Error
Error Code Returned
Communication
Counter Increments
Illegal parameter to read
Illegal Address (02h)
Invalid Address
Illegal enumeration in index
Illegal Data Value (03h)
Illegal Register
Table E.26
62h Read Enumeration Text Command
(Sheet 2 of 2)
Bytes
Field
Table E.28
7Dh Encapsulated Packet With Control Command
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)
1 byte
Embedded Modbus Function
n
bytes
Optional Data to Support Modbus Function (0–250)
2 bytes
CRC-16
A successful response from the slave will have the following format:
1 byte
Slave Address
1 byte
Function Code (7Dh)
2 bytes
Status Information (Register 2100h or 2101h
based on Bit 3 in Control Command Word)
1 byte
Embedded Modbus Function
n
bytes
Optional data to support the Modbus function (0–250)
2 bytes
CRC-16
Summary of Contents for 751A
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