Subject: Re: ISA IRQ signal active how long?
Date: Fri, 04 Oct 2002 20:23:28 -0400
Organization: Arius, Inc
NNTP-Posting-Date: 5 Oct 2002 00:23:34 GMT
X-Mailer: Mozilla 4.79 [en] (Win95; U)
Jim Stewart wrote:
> Norm Dresner wrote:
> >> You've not worked much with the ISA IRQ crap design, have you?
> > It's
> >>active HIGH. It's SUPPOSED to be totem-pole.
> >> Yes, it's crap. It's worse than crap, it's what happens when you
> >>feed crap to a horse - you get double-crap. But it IS. Sigh.
> >> (Yes, the proper way to handle IRQs is active-low level-sensitive
> >>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 OC
> > is the only way.
> > Norm
> Here's the way it used to be done:
> +5v |-
> | | -
> | | -
> |------| ------------ Bus IRQ
> | -
> | -
> |- O
> irq req/ _______|
> A light pulldown resistor on the output was often used.
To use this there had to be a pulldown and depending on the value, there
should only be ONE pulldown in the system. Since this was not part of
any standard and certainly not part of the ISA standard, there was no
pulldown on the motherboard. So to use this you had to have
non-standard boards and software.
Rick "rickman" Collins
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Arius - A Signal Processing Solutions Company
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