Modbus Exception Codes

The user may diagnose errors by examining response messages returned by the Modbus TCP Slave Interface to the Modbus master. Certain conditions will cause the Modbus TCP Slave Interface to return exception responses. This section documents the supported Modbus exception codes and their possible causes.

Exception Code


0x01 (Illegal Function)

Modbus request specifies an unsupported function code.

0x02 (Illegal Data Address)

Modbus request specifies an invalid data address, or specified a data address range that extends beyond the highest configured data address.

0x03 (Illegal Data Value)

Modbus write request specifies an analog data value that exceeds the data type range or the configured EU limits of the associated CIMPLICITY Point.

0x04 (Slave Device Failure)

Modbus write request specifies a data address mapped to a disabled, unavailable, or read-only CIMPLICITY Point.

The format for a Modbus exception response message is:

<slave address><function code><exception><CRC>


<slave address>

is the emulated slave device address.

<function code>

is one byte representing the failed operation. Note that the high bit is set.


is one byte representing the exception code. See table above. 


is trailing two bytes containing the cyclical redundancy check value.

More information

Modbus TCP Slave troubleshooting.