Weidmuller WI-I/O-9-U2: Wireless Mesh I/O & Gateway User Manual V1.2.2 - Manual (Page 117)

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Appendix D: Modbus Error Codes
The following are Modbus Error Response codes that can be read if utilising the Modus
mapping fail register and selecting a General Purpose Analog Register (30501, 40501,
etc.) instead of a General Purpose Digital register (10501, 501, etc.)
Dec
Code
Hex
Code
Name Meaning
65281 FF01
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.
65282 FF02
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, the PDU addresses the first register as 0, and
the last one as 99. If a request is submitted with a
starting register address of 96 with a quantity of 4
registers, then this request will successfully operate on
registers 96, 97, 98, 99. If a request is submitted with a
starting register address of 96 and a quantity of registers
of 5, then this request will fail with Exception Code 0x02
“Illegal Data Address”.
65283 FF03
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 significance of any
particular value of any particular register.
65384 FF04
Slave Device
Failure An unrecoverable error occurred while the server (or
slave) was attempting to perform the requested action.
65285 FF05
Acknowledge
Specialized, use in conjunction with programming
commands.
The server (or slave) has accepted the request and is
processing it, but a long duration of time will be required
to do so. This response is returned to prevent a timeout
error from occurring in the client (or master).