Sharing interrupts on the PC bus

[Marcus Hall]

I have seen several references to sharing interrupts in the Xenix instructions
about writing device drivers.  I am trying to set things up so I can share
interrupts, but it seems that the PC bus doesn't want me to do it.  The signals
are all active high, so they aren't driven with open-collector outputs.  Thus,
if one board isn't requesting an interrupt, it tries to hold the line low and
keeps any other board from trying to drive it high.  In practice, it seems
that the board with the stronger driver wins out and I get interrupts from
that board only.  This raises the question of any hardware out there that
has a COM3 or COM4 port installed and set to the default interrupt requests
4 and 3.  If that card is trying to drive the interrupt line as well as the
COM1 or COM2 card, how does that work?  Are people out there running systems
that only work in polled environments because of having COM3 or 4 equipment
plugged in?

On a related note, many of the COM3,4 cards can be set up to generate IR2 or
IR5 requests.  On an AT, IR2 is actually a cascaded input from a second
interrupt controller.  Does the IR2 pin on the bus go anywhere in an AT?
Perhaps one of the interrupts on the cascaded controller?

Thanks for any answers and/or advice...

marcus hall
..!mcdchg!illusion!marcus	marcus at illusion.UUCP

p.s.  Please email to me and if there really are alot of people who are
interested, I'll post the responses.