Path: utzoo!attcan!uunet!munnari.oz.au!bruce!monu1!vaxc!phs207k
From: phs207k@vaxc.cc.monash.edu.au
Newsgroups: comp.sys.ibm.pc.hardware
Subject: HELP NEEDED WITH IBM AT (286) INTERRUPTS!!!
Message-ID: <46762.26f56b86@vaxc.cc.monash.edu.au>
Date: 17 Sep 90 14:34:14 GMT
Organization: Computer Centre, Monash University, Australia
Lines: 39


I've  been  trying  to  build  some  hardware external to an AT and I need this
hardware  to  interrupt  the pc at a fairly high frequency (approx 3000Hz). Now
looking at the mother board manual for the 80286 I found interrupt lines 10, 11
and  12 are free on the slave 8259A PIC (pins 3,4 and 5 on the D side of the 16
bit expansion slot, although I only need two).

So  I  stuck  a wire in pin 4 (IRQ11) that is active high when interrupting and
wrote  a  masm  routine that is jumped to when the interrupt is accepted by the
8259A  programmable  interrupt  controller (that is contained within the 82c206
Integrated  Peripheral  Controller).  Ok,  I have set up an interrupt vector at
0:1ccH  in  memory  (the table for the slave 8259A) and I have sent commands to
the  8259A slave via ports A0H and A1H to unmask the interrupt lines I want to
use (OCW1).  Do I need to send anything to OCW2 for end-of-interrupt ? or is
the default mode ok.

Now my outside hardware sends a pulse (that stays high because I haven't worked
out  a way yet for the computer to kill it once it has recognized the interrupt
so  I  have  to do it manually) and sure enough the pc stops executing the main
program  and  jumps  off and executes the interrupt routine and even returns to
the main program.

PROBLEM  :  After the interrupt has finished the computer refuses to accept any
more  interrupts  from  my  hardware.  Even  if  the  main  program  terminates
successfully,  on  going  back  into DOS and trying to use the hard disk (which
uses  IRQ  14  on  the same 8259) the computer crashes. Now multiple interrupts
from  my  stuff shouldn't really be a problem because the interrupt enable flag
is  cleared  automatically  in  the  interrupt routine. I'm basically trying to
understand  what  has been affected to stuff things up when the interrupt has
been executed and comes back to the main program.  The setting up of the
actual int. vectors and the int. mask and checking IS and IR registers in the
8259 do not worry it (ie. I can run the setup program and not crash anything as
long as I do not do an actual interrupt).  So what is the trick?  I have
followed all the instructions I can find and tried everything I can think of. 
What is so hard about getting something outside the PC to interrupt it, run a
little program, and then go on with what it was doing?

I you need more information then just mail me and I'll be happy to pour out my
troubles to you but for me time is running out.  HELP!