Schweitzer Engineering Laboratories SEL-749M - Table D.2 SEL-749 M Modbus Function Codes; Table D.3 SEL-749 M Modbus Exception Codes

D.2
SEL-749M Relay Instruction Manual Date Code 20080918
Modbus RTU Communications Protocol
Communications Protocol
The SEL-749M SLAVEID setting defines the device address. Set this value to
a unique number for each device on the Modbus network. For Modbus
communication to operate properly, no two slave devices may have the same
address.
The cyclical redundancy check detects errors in the received data. If an error is
detected, the relay discards the packet.
Modbus Responses
The slave device sends a response message after it performs the action the
query specifies. If the slave cannot execute the query command for any reason,
it sends an error response. Otherwise, the slave device response is formatted
similarly to the query and includes the slave address, function code, data (if
applicable), and a cyclical redundancy check value.
Supported Modbus
Function Codes
The SEL-749M supports the Modbus function codes shown in Table D.2.
Modbus Exception
Responses
The SEL-749M sends an exception code under the conditions described in
Table D.3.
In the event that any of the errors listed in Table D.3 occur, the relay assembles
a response message that includes the exception code in the data field. The
relay sets the most significant bit in the function code field to indicate to the
master that the data field contains an error code, instead of the required data.
Table D.2 SEL-749M Modbus Function Codes
Codes Description
03h Read Holding Registers
04h Read Input Registers
06h Preset Single Register
10h Preset Multiple Registers
60h Read Parameter Information
61h Read Parameter Text
62h Read Enumeration Text
7Dh Encapsulate Modbus Packet With Control
7Eh NOP (can only be used with the 7Dh function)
Table D.3 SEL-749M Modbus Exception Codes
Exception
Code
Error Type Description
1 Illegal Function Code The received function code
is either undefined or unsupported.
2 Illegal Data Address The received command contains an
unsupported address in the data field.
3 Illegal Data Value The received command
contains a value that is out of range.
4 Device Error The SEL-749M is in the
wrong state for the function a query specifies.
This also stands for Service
Failure for DeviceNet interface
applications. The relay is unable to
perform the action specified by a query
(i.e., cannot write to a read-only register).
Courtesy of NationalSwitchgear.com