User`s guide

Page 123

Controlinc 1746-C (Version 5.21) Network Master Users Guide (2004-11-18)

6.5.3. Modbus Exception Messages Supported

There are 3 Modbus Exception Messages supported by the CPU (as responses back to the Modbus

master) for Modbus function calls to these registers:

• Exception 01  Illegal function code received. The query is not an allowable action for the

slave.

• Exception 02  Illegal data address received. The query is not an allowable address for

the slave.

• Exception 03  Illegal data value received. The value contained in the query field is not an

allowable value for the slave.

This is a 5 byte packet whose structure is always the same such that

• Byte 1: The slave address responding

• Byte 2: The function code that generated the error (+ 0x80)

• Byte 3: The error exception type (01, 02 or 03)

• Byte 4: The CRC high byte

• Byte 5: The CRC low byte

For byte #2 (function code generating the error), the most significant bit of the original function code that

generated the error/exception response is set to “1” to indicate which code had an error. For example:

• If function 1 (0x01) was sent to the slave and the slave had an exception with that function, it

would return a 0x81 in byte #2.

• If function 16 (0x10) was sent to the slave and the slave had an exception with that function, it

would return a 0x90 in byte #2.

Any “incorrectly formatted” message will return an Exception Code 02.