Allen-Bradley 825-P - 7 Dh Encapsulated Packet with Control Command

Rockwell Automation Publication 825-UM004D-EN-P - November 2012 143
Modbus RTU Communications Chapter 10
7Dh Encapsulated Packet
With Control Command
The 825-P uses this function to allow a Modbus master to perform control
operations and another Modbus function with one query. This command will be
transmitted periodically to achieve high-speed I/O processing and also serve as a
heartbeat between the communication option card and the main board.
The format of the relay responses to errors in the query is shown in Table 82.
Table 82 - 7Dh Encapsulated Packet Query Errors
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Subfunction (2000h)
1 byte 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 Subfunction (2100h or 2101h)
1 byte Function Code (7Eh)
n bytes Optional Data to Support Modbus Function (0…250)
2 bytes CRC-16
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Subfunction (2100h or 2101h)
1 byte Function Code (7Eh)
1 byte Function Error Code
2 bytes CRC-16