As per my earlier post, download the utilities I noted. These can send/receive modbus and give you hex dumps ofthe data they receive.

If you receive a packet with a bad CRC - it gets rejected silently.
If you send a packet to a slave ID that is not present you will also get no response.
If you send a packet with the wrong parity, it too will get silently rejected. A slave device can't respond to a corrupt packet since how does it know it is to be addressed? You only get an error response when a correctly formed packet is received but there is an error in the request - bad command, illegal register etc.


You've given us little hard evidence to diagnose your problem - just a lot of assumptions. Most likely one or many of your assumptions are incorrect and conspire to the problem you're experiencing.

With Modscan you can have it act as a master so you can set up a request the same as your micro and look at the data it sends to confirm the CRC etc. This feature alone has saved me an enormous amount of debugging time.