I am facing a bit of a problem.

The modbus slave I am trying to communicate with requires a 16bit CRC, which doesn’t seem that unusual.

However, in the ‘help’ file for the CB1241 it seems to suggest that a CRC is not included in the message frame.

I have found a block in the library called Set_Features.

The help file appears to suggest that this will enable a CRC with modbus

I’ve tried it a few different ways without an error code but I can’t see a CRC being added to the message frame. I note that in the description it says “if supported by the module” – does anyone know how I find out which modules are supported? How is the CRC calculated?

Your graphic illustration just omits (leaves off) the CRC from the end of the Modbus message. 

If it claims to be Modbus RTU, then it has to have a 16 bit CRC.

The graphic in the link below, from some other device (maybe even the Modbus spec), also for Function Code 03, shows the same message format with the CRC at the end.

[snip]

 Hi,

The illustration was from the TIA Portal v15 help (F1) file. I checked the message being sent with a windows Modbus slave and no CRC.

There is another block called Get Features, which indicated the CB supported CRCs.

I called Set Features once at start up and this seems to have resolved the issue.

danw

Your graphic illustration just omits (leaves off) the CRC from the end of the Modbus message. 

If it claims to be Modbus RTU, then it has to have a 16 bit CRC.

The graphic in the link below, from some other device (maybe even the Modbus spec), also for Function Code 03, shows the same message format with the CRC at the end.

[snip]

 Weird, maybe I missed it the first time around. 

Kaulquappe

Hello,
the FBs MB_MASTER and MB_SLAVE send a CRC for sure.

Regards,
Kaulquappe