Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!tektronix!sequent!mntgfx!gssc!geoffs
From: geoffs@gssc.UUCP (Geoff Shapiro)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Mysterious Interrupt
Message-ID: <5920@gssc.UUCP>
Date: 21 Apr 89 17:12:34 GMT
References: <7970@pyr.gatech.EDU>
Reply-To: geoffs@gssc.UUCP (Geoff Shapiro)
Organization: Graphic Software Systems, Beaverton Or
Lines: 29

In article <7970@pyr.gatech.EDU> is813cs@pyr.gatech.edu (Cris Simpson) writes:
>I am developing an interrupt driven data collection system running
>on a Compaq 386-25.  It works fine, except that I am getting
>tripped up by an unknown interrupt, which makes my latency high (1.8ms).
>It occurs at about 230Hz.  Can anyone suggest what this might be?
>I'm getting 6000 ints/sec except when I'm in the mystery int?
>

What kind of hardware are you using to do the data collection? Perhaps an
A-to-D card which is generating interrupts at a frequent rate? The reason that
I ask is because I have seen similar behaviors here when running cards in
the bus which are generating too much noise on the interrupt request lines
of the bus.

To understand this you must understand the 8259 and how it handles noisy
interrupt input lines. A little known feature of the 8259 is that when noise
generates a pulse on one or more of any of the interrupt input lines and this
pulse is not of the length or level required for recognition as an unqualified
interrupt source, the 8259 will internally trigger an IR7. Normally, in the
DOS system IR7 is vectored to a null interrupt service handler and therefore
is blown off harmlessly except for the degradation of system throughput
when the lines are real noisy.

I would check this out as a possible source of your problems...

...!{tektronix!verdix}!sequent!gssc!geoffs             Geoff Shapiro
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