11.8.11 Modbus Exception Codes
For a full explanation of the structure of an exception code response, please refer to the section
Modbus RTU Message Framing Structure, Function Field.
Modbus Exception Codes
Code
Name
Meaning
1
Illegal function
The function code received in the query is not an allowable action for the server (or slave). This may
be because the function code is only applicable to newer devices and was not implemented in the unit
selected. It could also indicate that the server (or slave) is in the wrong state to process a request of
this type, for example because it is not configured and is being asked to return register values.
2
Illegal data address
The data address received in the query is not an allowable address for the server (or slave). More
specifically, the combination of reference number and transfer length is invalid. For a controller with
100 registers, a request with offset 96 and length 4 would succeed, a request with offset 96 and length
5 will generate exception 02.
3
Illegal data value
A value contained in the query data field is not an allowable value for server (or slave). This indicates
a fault in the structure of the remainder of a complex request, such as that the implied length is
incorrect. It specifically does NOT mean that a data item submitted for storage in a register has a value
outside the expectation of the application program, since the Modbus protocol is unaware of the sig-
nificance of any particular value of any particular register.
4
Slave device failure
An unrecoverable error occurred while the server (or slave) was attempting to perform the requested
action.
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11 RS-485 Installation and Set-up
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