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From: roe@unibase.uucp (Roe Peterson)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: sharing hardware interrupts
Message-ID: <1991May31.154808.5436@unibase.uucp>
Date: 31 May 91 15:48:08 GMT
References: <1991May29.014824.16278@ux1.cso.uiuc.edu> <1991May30.125133.527@syd.dit.CSIRO.AU>
Organization: Unibase Telecom Inc.
Lines: 32

evans@syd.dit.CSIRO.AU (Bruce.Evans) writes:

>In article <1991May29.014824.16278@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>>Suppose you have two or more different serial ports in a PC.
>>When any one of the serial ports causes an interrupt (for
>>the sake of discussion let's say it is #4 and nothing else uses #4)
[...]
>>Given the above software, why could it not be possible to have two or
>>more devices share the same interrupt.

>Because if 2 devices are driving the same interrupt line, the one driving
>it low wins. For some reason the PC interrupt h/w is designed so that
>low == no interrupt. This is the reverse of what is needed for easy
>interrupt sharing.

True - however, I have successfully shared an interrupt with two
(and only two) serial ports.  It seems that _some_ hardware can drive
an interrupt line high even when one other port is holding it low.
Attempts to share three ports on one interrupt fail.

The hardware I've used is the MCTAMS 4-port serial card (we get them from
JDR Microsystems), with the FAS 2.08.0 device drivers under ESIX 5.3.2.D.
However, since sharing interrupts is more of a hardware question, you should
be able to get away with the same sort of setup under DOS (given a serial
device driver capable of the right magic.)

-- 

						  Roe Peterson
						  uunet!sobeco!unibase!roe
						  roe@unibase.UUCP