I have a PCB with an atmega8, communicating with a PC. The signal from TX and RX pins is level-shifted through a MAX232 to standard serial port level, then converted via serial-to-USB commercial device, which connects to the PC. The PCB with the atmega8 and the PCB, holding the MAX232 are powered from the same brick (transformer-based power supply).
Do I connect the GND of MAX232 pcb to ground of the serial port (which goes to the PC)? Do I connect the GND of the atmega8 board to GND on the MAX232 board? I would prefer that it doesn't matter if the two boards are powered from the same PSU or separate.