Thanks for the suggestions, and I apologise, I couldn't attach links.
Yes I know these serial communication pins and have tried with TX1-RX1 to TX3-TX3 pins but none of them worked.
The both boards (UNO and OpenCM) are powered through their USB ports, therefore there is no any connection of voltage between them. Only he GND, TX and RX pins are connected between them to transfer the data from one board to the other.
I really don't have any idea why the data is not transferring, still struggling with it. and thanks again.