User manual

Modbus Commands

2-38

SIMATIC TI505/TI500 MODNIM User Manual

2.5 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 Function Code

with the top bit set high (see examples in Figure 2-34 and Figure 2-35).

07 00000041 AD

Address Function Data LRCStart EOF

 01 CR LF

Ready

Request:

07 02 76

Address Function Except.

Code

LRCStart EOF



81 CR LF

Ready

Response:

Figure 2-34 Exception

Response Frame— ASCII Mode

07 01 00000041 XXXX

Address Function Data CRC

Request:

07 81 02 XXXX

Address Function Exception

Code

CRC

Response:

Figure 2-35 Exception

Response Frame — R

TU Mode

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.