RS232.2
Other notes:
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AT class computers have more IRQ lines than XT class computers.
IRQ 2 has been said to be very important in an AT computer so you might
want to avoid using this one until the very last.

What I have in my com board is that there are traces running to B24 and
B25 but none to B4,B23,B22 and B21. This is standard for most com boards
. I already had these IRQ lines used by the multifunction card of my
XT, so on my com board, I taped over B24 and B25; set dip switches on
com board for an address of 3e8; soldered a wire from B24 to B2;
configured the msys.opt file for 3E8 IRQ4, tnctype... (0 for a KPC2)
fired MSYS up and it ran!
  The previous note on using debug to poke memory was not needed. I'm
sure this setup will work with phone modem accesses and extra ports
that you might want to add. Also, the cost of a com board is relatively
cheap. Here in Victoria, they run $25 with 1 port of 2 installed.
  The second com port (future com4) is socketed for an extra 8250
1488, and 1489 chip set. If you get the buffered 8250, you can run
higher speed packet if the unbuffered 8250 boggs your system down.
  The theory behind this according to the technician is that a regular
8250 waits for 2 bytes before flagging an interrupt, while a buffered
8250 will wait for 16 bytes before doing same, thus interrupting your
processor less often - more efficient.
    73 - Larry, VE7DIE @ VE7DIE.#vic.bc.ca