From owner-freebsd-hardware  Thu Jul 18 09:03:31 1996
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Date: Thu, 18 Jul 1996 11:08:33 -0500 (CDT)
From: Alex Nash <alex@fa.tdktca.com>
To: E00114@vnet.atea.be
cc: freebsd-hardware@freebsd.org
Subject: Re: Multiple COM ports with same IRQ
In-Reply-To: <vines.mmb7+gqYvlA@vnet.atea.be>
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On Thu, 18 Jul 1996 E00114@vnet.atea.be wrote:

> ISA boxes can 
> have hardware installed (accidentally?) with devices sharing the same IRQ, but 
> the drivers cannot deal with it.

The sharing of interrupts between ISA devices is an electrical 
constraint.  When two devices are attempting to use a common IRQ line, 
the devices compete with each other (electrically) to put the line in 
their desired state.  Needless to say, the results are less than 
optimal.  It is possible for the devices to share an interrupt if both 
devices can tri-state their IRQ lines.  That way when you're operating 
device A, device B's IRQ line has no pull on the line, and vice versa.  
Naturally this precludes you from using both devices simultaneously 
(and even if the hardware does support tri-stating the IRQ line, the 
software may not activate it).  [This only deals with ISA cards, the 
operation of on board serial ports are implementation specific.]

Alex