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From: "Meindert Sprang"
Subject: Re: ISA IRQ signal active how long?
Date: Fri, 11 Oct 2002 08:30:18 +0200
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"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
> 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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