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From: "Meindert Sprang"
Subject: Re: ISA IRQ signal active how long?
Date: Fri, 11 Oct 2002 08:34:45 +0200
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Shoot!. I must learn to read before writing.
The Intel ISA spec is something else. It is, and I quote: "a parallel bus
derived from the IBM PC/AT memory and I/O bus". Probably a step-up to the
"Meindert Sprang" wrote in message
> "Noname Nospam" wrote in message
> > The usual driver for for an IRQ line is the 74LS125, Tri-State buffer.
> > The IRQ for the ISA bus are ACTIVE HIGH.
> > The 8259 PIC for DOS and Windows is programmed for LEVEL sensitive mode.
> Is that what it says in the IBM manual you mention below? I have here an
> Intel ISA bus spec which says that 'the interrupt controller will react to
> the interrupt on a transition of low to high'. Futher in this document it
> also says that the interrupts are edge triggered.
> It seems that Intel and IBM are not at the same wavelength here....
> > The IRQ remains asserted untiled the interrupt source is serviced by the
> operaing system.
> > The original IBM-PC never intended to share IRQ lines.
> > When IBM introduced the AT they tried to define a method to share IRQs.
> > The "offical" specification can be found in the:
> > "IBM Technical Reference Personal Computer AT"
> > (6280070, S299-9611-00), Revised edition (March 1986)
> > Section 1, pages 14 through 21.
> > This method was never widely adopted. Possibly because all the current
> > I/O cards would have to be replaced by ones that could "share" the IRQ.
> > At the time the S-100 bus used an active low, open collector, IRQs.
> > This means that IBM didn't copy something that worked they had to invent
> > a way to do it wrong. That is why even today 24 years later we are still
> > with the cheap a** active high IRQs that some IBM newbie bone head
> > engineer came up with.
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