A violation regarding the specifications for the Read Exception Status.

[ProtocolSpecCheck]

Hi,

I noticed a violation regarding the specifications for the Read Exception Status.

In the MODBUS APPLICATION PROTOCOL SPECIFICATION V1.1b3, there is one specification of Read Exception Status in the Page 21:

Precondition - !(Function code support)

Postcondition - ExceptionCode == 0x01

However, libmodbus does not return the expected response with "ExceptionCode == 0x01." Instead, it provides no response.

Steps to reproduce(after installation):

1）Terminal-1 (Server: bandwidth-server-many-up)
./libmodbus/tests/bandwidth-server-many-up

2. Terminal-2 (Client: sends a request message with the function code for Read Exception Status)
   b"\x00\x01\x00\x00\x00\x02\xff\x07"

3. Observe and found that there is no response from the server.

Related Code:

libmodbus/src/modbus.c

Line 1072 in bb13443

case MODBUS_FC_READ_EXCEPTION_STATUS:

Thank you for your attention.

[ProtocolSpecCheck]

Hi, @mhei @stephane

We have submitted a PR to address this issue. Do you think it is reasonable?

• PR: (RFC-compliance: Issue813) A violation regarding the specifications for the Read Exception Status. #833

Thank you for your attention, and please let me know if you have any questions or need further information.

Looking forward to your reply : )