Schweitzer Engineering Laboratories SEL-749M - Table D.21 7 Dh Encapsulated Packet Query Errors; Table D.22 7 Eh NOP Command

D.9
Date Code 20080918 Instruction Manual SEL-749M Relay
Modbus RTU Communications Protocol
Communications Protocol
The format of the relay responses to errors in the query is shown in
Table D.21.
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 required.
Reading Parameter
Information and Value
Using Modbus
Through use of Modbus commands, you can read the present value of a
parameter as well as parameter name, units, low limit, high limit, scale, and
even the enumeration string (if the parameter is an enumeration type). This
means that you can use a general user interface to retrieve and display specific
parameter details from the relay. Use the 60h, 61h, and 62h commands to
retrieve parameter information, and use the 03 command to retrieve values.
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 D.21 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 D.20 7Dh Encapsulated Packet With Control Command (Sheet 2 of 2)
Bytes Field
Table D.22 7Eh NOP Command
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
Courtesy of NationalSwitchgear.com