The best way to test the serial TX and RX data lines, is get a terminal program, use any baud rate, and disable flow control and disable local echo.
Link the TX & RX data together (pins 2 & 3 on serial port) and type something on the keyboard, which should echo to the screen then stop echoing when you remove the TX & RX link.
If this works OK, and you still get 1 way data to the other computer, use software flow control (or no flow control for testing). If this works, there is a problem with the hardware flow control lines, either in one of the computers, or in the null modem cable.
If the linked TX & RX test doesn't work, there is possibly a problem with the +12/-12V supply (required for RS232 transmission), the 1488/1489 line driver/receiver or the CIAs.
