From: richard AT stardate DOT bc DOT ca
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Detecting interrupt on printer port
Date: Wed, 19 Aug 1998 04:02:07 GMT
Organization: Rapidnet Technologies Internet
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On Wed, 19 Aug 1998 01:01:19 GMT, "Michel Gallant"
<mgallant AT NOSPAM DOT grassroots DOT ns DOT ca> wrote:

>
>Fabian Nunez wrote in message <6rba5l$b9v$1 AT groa DOT uct DOT ac DOT za>...
>
>>??? Are you sure about that?  I'm pretty sure that '1' on port 21h
>>means "mask out this IRQ"... (although I'm in the lab at uni, and my
>>inter57 is sitting several K's away at home)

Yup I got it backwards.  

>
>Fabian, you are right.  I looked it up myself in a reference manual I
>have.  I did incorporate an interrupt-unmasking routine into my
>program (CLEARING bit 7 of 0x21)  and my program still doesn't work.
>Any other suggestions?  Thanks.

One thought strikes me. Interrupt 7 has the lowest priority, meaning
all other interrupts get seviced first.  Is it possible that by the
time the interrupt has a chance to be serviced, the acknowledge line
has gone high again and the interrupt is never called? Interrupt 15,
that you have been catching should have a high priority because it is
a hard drive interrupt.  I don't think the 8259 latches its inputs. 

Have you tried connecting the acknowledge line to ground with a 1-2k
resistor. This would force the interrupt to be on constantly and
should get a responce, even if only once.  Better yet would be a 555
time set up as an oscilator, then you can tweek both frequency and
the percent of the frequencys period that is low.  Just about any
osilator that uses an RC network to control the frequency would work.

If all else fails you could have a programme loop calling interrupt 7.
This is not a normal situation but it can be done.