Schweitzer Engineering Laboratories SEL-751 - Table E.31 7 Dh Encapsulated Packet Query Errors; Table E.32 7 Eh NOP Command

E.16
SEL-751 Relay Instruction Manual Date Code 20170927
Modbus RTU Communications
Communications Protocol
Table E.31 shows the format of the relay responses to errors in the query.
7Eh NOP Command
This function code has no operation. This allows a Modbus master to perform
a control operation without any other Modbus command. This is only used
inside of the 7Dh when no regular Modbus query is necessary.
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
Table E.31 7Dh Encapsulated Packet Query Errors
Bytes Field
Queries from the master must 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 Modbus Function with Error Flag
1 bytes Function Error Code
a
a
If the embedded function code is invalid, then an illegal function code is returned here and
the illegal function counter is incremented. This error code is returned by the embedded
function for all valid embedded functions.
2 bytes CRC-16
Table E.30 7Dh Encapsulated Packet With Control Command (Sheet 2 of 2)
Bytes Field
Ta b l e E . 32 7 E h N O P Co m m an d
Bytes Field
An example of a 7D message response using 7E will have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Status Information
1 byte Function Code (7Eh)
2 bytes CRC-16