From: "Ralph Wade Phillips"
Subject: Re: ISA IRQ signal active how long?
Date: Fri, 4 Oct 2002 19:46:18 -0500
Organization: Phillips Enterprises
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"Norm Dresner" wrote in message
> > You've not worked much with the ISA IRQ crap design, have you?
> > active HIGH. It's SUPPOSED to be totem-pole.
> > Yes, it's crap. It's worse than crap, it's what happens when
> > feed crap to a horse - you get double-crap. But it IS. Sigh.
> > (Yes, the proper way to handle IRQs is active-low
> > with open-collector drivers. Not how IBM did it, however.)
> Regardless of your opinion of the design, it is what it is.
> Also, if the interrupt is to be shared -- and it *does* work -- then
> is the only way.
OK. So - you have device A and device B both using IRQ 4, say.
Both are OC devices. So, both turn on the outputs so that they can pull the
line low (remember - PC ISQ IRQs are active-high edge-triggered!).
Exactly HOW does device A signal an IRQ? Device B has the line held
LOW - so there's no edge to trigger the PIC.
"Well, reset it!"
And promptly screw up OTHER cards on the ISA bus.
I don't disagree that OC is the way to go for shared IRQs. But if
you go OC, you just destroyed ANY possibility of having the IRQ work right
if it IS shared ... TriState, and activate ONLY when not being IRQed is the
way to do it. That, or forget ISA and use something with a rational
interrupt design ...