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Re: [aclug-L] Modem configuration question

Re: [aclug-L] Modem configuration question

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To: aclug-L@xxxxxxxxxxxx
Subject: Re: [aclug-L] Modem configuration question
From: "Brian J. Chapman" <bchapman@xxxxxx>
Date: Thu, 15 Oct 1998 21:02:03 -0500
Reply-to: aclug-L@xxxxxxxxxxxx

At 04:36 PM 10/15/98 -0500, Dale W Hodge wrote:
> I beg to differ with you.  Here's my interrupts:
> Irq 2 is clearly the cascade, and irq 9 is a valid irq.  My
> was to avoid using 2, as 9-15 are linked through it. 

Yes thats correct. The IRQ2 lead on the master 8259A interupt controller
chip on the mother board receives the output (cascade) from IRQ 9 on the
slave 8259A. This is how we have IRQs 9-15 to begin with. IRQ9 on the
slave 8259A chip is wired directly to the motherboard and the output is
directed to IRQ2 on the master 8259A chip. So IRQ9 on the slave is what
receives the actual signal from the hardware, but it isn't read until it
flows over into IRQ2 where it is then detected by the master interrupt

As an excersise to the reader, look on any configurable modem and you'll
see jumpers for IRQs 2,3,4,5, and 7 but never 9. That's proabably
left-over thinking from the XT days, perhaps the hardware/software knows
the difference and redirects it accordingly.

Anyway, I was just trying to explain to Greg the reason why `setserial'
must have kept insisting upon using IRQ2 even when he specificaly told it
to use IRQ 9. 

<*Sigh*> I'll just be glad when everything goes to USB!

 Brian James Chapman | bchapman@xxxxxxxxx |
  "Absolutely nothing should be concluded from these figures except
   that no conclusion can be drawn from them."
                         -- Joseph Brothers, Linux/PowerPC Project

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