From owner-freebsd-hardware  Fri Jul 19 14:42:46 1996
Return-Path: owner-hardware
Received: (from root@localhost)
          by freefall.freebsd.org (8.7.5/8.7.3) id OAA21378
          for hardware-outgoing; Fri, 19 Jul 1996 14:42:46 -0700 (PDT)
Received: from cbgw1.att.com (cbgw1.att.com [192.20.239.133])
          by freefall.freebsd.org (8.7.5/8.7.3) with SMTP id OAA21371
          for <freebsd-hardware@freebsd.org>; Fri, 19 Jul 1996 14:42:41 -0700 (PDT)
Received: from aloft by cbig1.att.att.com (SMI-8.6/EMS-1.2 sol2)
	id RAA01471; Fri, 19 Jul 1996 17:30:25 -0400
Received: from stargazer (stargazer.cnet.att.com) by aloft (4.1/DCS-aloft-M5.1)
	id AA16133; Fri, 19 Jul 96 17:35:13 EDT
Received: by stargazer (4.1/DCS-aloft_client-S2.1)
	id AA15984; Fri, 19 Jul 96 17:35:10 EDT
Date: Fri, 19 Jul 96 17:35:10 EDT
From: gtc@aloft.att.com (gary.corcoran)
Message-Id: <9607192135.AA15984@stargazer>
To: Brett_Glass@ccgate.infoworld.com, alex@fa.tdktca.com, bde@zeta.org.au
Subject: Re: Multiple COM ports with same IRQ
Cc: E00114@vnet.atea.be, freebsd-hardware@freebsd.org
Sender: owner-hardware@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk


>> maybe you can tell me how an EISA/ISA box solves the IRQ sharing problem
>> for ISA devices.
>
>It doesn't. Only EISA devices or E-ISA devices can share IRQs.

To clarify, (correct me if I'm wrong):
EISA devices, which by definition only plug into an EISA motherboard,
can only share IRQs if they have "open-collector" (i.e. open-drain)
drivers on the IRQ line.  This allows any device on a particular IRQ to
pull the line low without "fighting" any other device's IRQ driver.
This capability, combined with the fact that EISA motherboards allowing
changing the PIC operation to be level sensitive instead of edge-triggered,
is what potentially allows EISA devices to share IRQs.

One question though: The "open-collector" scheme described above (which is
what is commonly used for interrupt sharing) works quite well for 
ACTIVE LOW interrupt signalling.  But the IRQ lines on EISA (and ISA)
are ACTIVE HIGH signals.  Do EISA PICs also allow the active state of
the bus IRQs to be changed from high to low? (I don't think so).

If not, then how *is* IRQ sharing implemented on EISA devices?

Gary

Gary.Corcoran@lucent.com