i don't have A2NCPU laying around so I can't help much.
however, any port will have appropriate driver circuitry
and A2NCPU can't be exeption. if i was going to try to fix it i would look into following:
- part numbers of all chips and get the datasheets
- any chip that is connected to port
for example RS232 devices will have some sort of RS232 driver,
RS422/485 ports will use some sort of line driver chip.
although there is tons of them, there are only few actual variations as they are mostly drop in replacements etc.
you can check online electronics stores or just google to identify "line driver" chips.
if the circuitry is SMT (probably is at leat in part even on older devices), practice soldering on something else, regadless if it works or not (anything with components of right size just to learn how to do it right, any old motherboardsm HDDs, CD drives etc are good candidates).
in some cases pcb traces are burnt and need to be repaired/replaced as well. i use enameled wire (you can buy it or just canibalize relay or small transformer). this also needs exercise, before you even think of using it on PCB, you need to verify if you can reliably and repeatably "strip" the insulation (enamel) etc. (for signals i use very thin, hair like wire so it can be soldered to smallest pins or vias. small size makes it next to invisible so it doesn't leave much clutter and it's easy to "strip" - just dip it into small drop of solder and enamel will go in smoke)
good luck