Sel 551 - Supported Modbus Function Codes; Modbus Exception Responses

G.2
SEL-551 Relay Instruction Manual Date Code 20110408
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
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
requested in the query. If the slave cannot execute the command for any
reason, it sends an error response. Otherwise, the slave device response is
formatted similarly to the query including the slave address, function code,
data (if applicable), and a cyclical redundancy check value.
Supported Modbus
Function Codes
The SEL-551 supports the Modbus function codes shown in Table G.2.
Modbus Exception
Responses
The SEL-551 sends an exception code under the conditions described in
Table G.3.
In the event that any of the errors listed in Table G.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 requested data.
Cyclical Redundancy
Check
The SEL-551 calculates a 2-byte CRC value using the device address,
function code, and data fields. It appends this value to the end of every
Modbus response. When the master device receives the response, it
recalculates the CRC. If the calculated CRC matches the CRC sent by the
Table G.2 SEL-551 Modbus Function Codes
Codes Description
01h Read Coil Status
02h Read Input Status
03h Read Holding Registers
04h Read Input Registers
05h Force Single Coil
06h Preset Single Register
07h Read Exception Status
08h Loopback Diagnostic Command
10h Preset Multiple Registers
64h Scattered Register Read
Table G.3 SEL-551 Modbus Exception Codes
Exception
Code
Error Type Description
01 Illegal Function Code The received function code is either undefined or
unsupported.
02 Illegal Data Address The received command contains an unsupported
address in the data field.
03 Illegal Data Value The received command contains a value that is out
of range.
04 Device Error The SEL-551 is in the wrong state for the requested
function.
06 Busy The SEL-551 is unable to process the command at
this time due to a busy resource.