Sel 751A - Table E.27 61 H Read Parameter Enumeration Text Query Error Messages; Table E.28 7 Dh Encapsulated Packet with Control Command

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