Error Responses; C.4 Error Responses; C-37 Exception Response Frame; Exception Responses - Siemens SIMATIC TI500 User Manual

C.4

Error Responses

C-28 Modbus Commands
Errors such as illegal addressing or failure of communications with a PLC
are reported by exception responses. Exception responses replace the
expected response to a request and are recognized by the returned Function
Code with the top bit set high (see example in Figure C-37).
Request:
Address
07
Response:
Address
07
Figure C-37 Exception Response Frame
In this example, the request is for secondary node 07 to return the data for
65 (41 hex) coils starting at address 00, which is an illegal address for a
SIMATIC TI PLC. The response is an exception indicated by the Function
Code 81 (01 with the high bit set), with the exception code 02 indicating that
the data address was illegal.
NOTE: The TIWAY I network uses Exception Primitives. Exceptions sent by
SIMATIC TI PLCs are mapped to Modbus Exception Codes.
Table C-6 list the exception responses that apply to the functions supported
by the TIWAY I Gateway.
Code Name
01 Illegal Function
02 Illegal Data Address
03 Illegal Data Value
Function Data
01 00000041
Function Exception Code
81
Table C-6 Exception Responses
The function received is not defined for this
application [equivalent to SIMATIC TI
exception codes 00, 01, 06, 15].
The address contained in the data field is not
valid for the secondary being addressed
[equivalent to SIMATIC TI code 02]
The value passed in the data field is not
allowable for the secondary being addressed
[equivalent to SIMATIC TI codes 10, 19, 1D].
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Description
TIWAY I Gateway User Manual
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