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Modbus Commands

C-28

TIWAY I Gateway User Manual

C.4 Error Responses

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).

07 01 00000041 XXXX

Address Function Data Checksum

Request:

I000000

07 81 02 XXXX

Address Function Exception

Code

Checksum

Response:

Figur

e 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

SIMA

TIC 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 TIW

Y I network uses Exception Primitives. Exceptions sent by

SIMA

TIC TI PLCs are mapped to Modbus Exception Codes.

able C-6 list the exception responses that apply to the functions supported

by the TIW

Y I Gateway

able C-6

Exception Responses

Code Name Description

01 Illegal Function The function received is not defined for this

application [equivalent to SIMATIC TI

exception codes 00, 01, 06, 15].

02 Illegal Data Address The address contained in the data field is not

valid for the secondary being addressed

[equivalent to SIMATIC TI code 02]

03 Illegal Data Value 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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