speedy wrote:So, I've tested ... sending data from any x-sim software with a string [ ab~255~~a01~~a02~ ] to the AMC com port ... and the LCD first left numbers for both lines are still fixed ...
I want to draw your attention that ab~255~~a01~~a02~ is not equal AB~255~~a01~~a02~. AMC wait string with "AB" header, not "ab".
And now about your communication problem. I think that is not Windows problem, and not Software problem.
SubZERO step - Make shure, that you use FW for your quartz frequency

Step ZERO - Shut down any communications programm on PC, set dip switch 1 to "off" position, connect AMC to PC. On LCD should be string that AMC in TTY mode. Then run 'terminal' program on PC, open COM-port. Make shure that COM-port setup is: 115200 bod, One stop bits, Non parity. Then try to reset AMC. After reset In the terminal window should appear message from AMC. If it absent, go to step 1
The first step - check, please, electrical connection between the TXD (and RXD) pin of FT232RL module and RX (andTX) pin of ATMEGA. Check solder quality. I hope, you have a multimeter to do it. If it done, go to step 2

The second step - if you can (if you have oscilloscope), check communication pulses on pin RX ATMEGA when xSimCTRL is run. If they are present go to step 3.
The third step - if you can (if you have oscilloscope), check quartz frequency. Because if it is different from the nominal ( 22.1184Mhz) more than on 1-2% there is sinhronization problem. It is possible if quartz works on another harmonica.
The next step -



The next step - ask help to Thanos

Regards,
Ale