Third serial port

[Doug Ingraham]

In article <1989Dec19.002448.2979 at virtech.uucp>, cpcahil at virtech.uucp (Conor P. Cahill) writes:
> In article <12978 at cit-vax.Caltech.Edu>, 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?
> > Is it possible to share interrupts (can I put two ports on the same
> > interrupt and have it work)?
> 
> 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.

Conner is correct.  But if you don't already know this its still confusing.

On an XT IRQ2 goes to IRQ2.  On an AT the IRQ2 pin on the bus goes to IRQ9.
The real IRQ2 is used to cascade the second interrupt controller chip. So
what you need to do is select IRQ2 on your card but tell Unix to use IRQ9.
Some cards are designed to allow the interrupts to be shared.  In those
cases you should be able to set both cards to interrupt 3, but I don't have
any experience with anyone who has made this work.  Another problem with 
using IRQ2 is that some of the EGA cards on the market think they own IRQ2
and weren't designed to allow sharing.  (The interrupt is for vertical
interval.)  You can cut the trace on the card or put a piece of tape across
it if you want to try using it.  I don't know of any applications that make
use of it.  It is pin B4 on the connector (32 pin).  There are actually
lots of interrupts available if you have a 16 bit card.  10, 11, 12, and
15 are listed as reserved by IBM.  Probably these are used by 16 bit
network boards but since I haven't had any direct experience with them I
can't be certain.  On your normal 8 bit com port you can select IRQ3 or
IRQ4.  If you dont use parallel ports you could also use IRQ5 or IRQ7.

Best Wishes



-- 
Doug Ingraham (SysAdmin)
Lofty Pursuits (Public Access for Rapid City SD USA)
uunet!loft386!dpi