From: Johnny Billquist <bqt@Update.UU.SE>

>For some obscure reason, IBM in all their usual wisdom (the same
>wisdom that gave you the PC ASCII) decided that they should use a DB9 on
>their PC, and while using RS-232 levels, should change pin 2 & 3 from what
>the RS-232 spec said, and also move the ground from pin 7 to pin 5.

Actually, they only switched to the DB9 on the AT (I believe the reason
was because otherwise they couldn't fit >1 connector on those nasty little
back-panel brackets).  The serial port option for the PC and XT had a DB25M
just like it should, and incidentally could do 20 mA if you switched a
jumper block around backwards.  Leave it to IBM to not even be compatible
with their own stuff 2 years later...

John Wilson
D Bit