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From: "Noname Nospam"
Subject: Re: ISA IRQ signal active how long?
X-Newsreader: Microsoft Outlook Express 5.00.2314.1300
NNTP-Posting-Date: Thu, 10 Oct 2002 20:01:20 EDT
Organization: Prodigy Internet http://www.prodigy.com
Date: Fri, 11 Oct 2002 00:01:20 GMT
Ru-Chin Tsai wrote in message news:email@example.com...
> I design a ISA card with IRQ support. We know 8259's IRQ signal is
> active high. I have read many posts about ISA IRQ pin and remain
> feeling some confusions. Please help me solve these problems.
> 1. The ISA IRQ will be totem-pole or open-collector or tr-state output type.
> It seem to have no standard about this issue. Many answers were posted on
> newsgroup and I can't identify which one is correct output configuration?
> 2. When ISA card request interrupt. It must assert the IRQ signal. But how long
> does IRQ signal be asserted(active high)?
> 3. Should I care the 8259 PIC act as edge-trigger or level-sensitive mode?
> Thank you for your help sincerely. These problem delay my project for long time.
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.
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 stuck
with the cheap a** active high IRQs that some IBM newbie bone head
engineer came up with.
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