Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD
April 2018
253
6.3.5
Communication Errors
Serial link communication errors are divided into three groups:
▪
Invalid Query Message
▪
Serial Link Time Outs
▪
Invalid Transaction
Invalid Query Message
When the communications module receives a query addressed to itself, but cannot process the query, it
sends one of the following error responses:
Subcode
Invalid Function Code
1
Invalid Address Field
2
Invalid Data Field
3
Query Processing Failure
4
The format for an error response to a query is as follows:
Address
Exception
Func
Error
Subcode
Error
Check
Figure 64: RTU Error Response Format
The address reflects the address provided on the original request. The exception function code is equal
to the sum of the function code of the query plus 128. The error subcode is equal to 1, 2, 3, or 4. The value
of the subcode indicates the reason the query could not be processed.
Invalid Function Code Error Response (1)
An error response with a subcode of 1 is called an invalid function code error response. This response is
sent by a slave if it receives a query whose function code is not equal to 1- 8, 15, 16, 17, or 67.
Note:
Starting with Release 6.70 for the RX3i, the invalid function code error response is not used.
Instead, undefined and unsupported function codes are ignored, and no response is
generated.
