Third serial port

[Dick Dunn]

cpcahil at virtech.uucp (Conor P. Cahill) writes:
> ...tim at cit-vax.Caltech.Edu (Timothy L. Kay) writes:
> > Does anybody know if it is possible to use IRQ 2 for a serial port?
> You can't use IRQ 2 for anything on any AT bus system.  IRQ 2 is used 
> by the second interrupt handler.  Usually IRQ2 on a bus card is mapped
> to IRQ 9, so try that or try a different IRQ.

IRQ2 on a PC corresponds to IRQ9 on the AT bus--which includes the 386 ISA
machines.  By that I mean that the line on the bus which generates an IRQ2
on a PC generates an IRQ9 on an AT (since, because of the reason Cahill
points out, it wouldn't make sense to have an IRQ2 bus line).  So, if you
have a board which is designed to have "IRQ2" as one of its choices,
selecting that will give you an IRQ9 on an AT-bus machine without further
ado.
-- 
Dick Dunn     rcd at ico.isc.com    uucp: {ncar,nbires}!ico!rcd     (303)449-2870
   ...Never offend with style when you can offend with substance.