💾 Archived View for uscoffings.net › retro-computing › systems › Tandy › t › whichirq.txt captured on 2022-06-04 at 00:51:15.
-=-=-=-=-=-=-
Fm: Jim Unger 72747,1217
To: Wayne Day 76703,376
Wayne,
I took your advice of a couple days ago and checked the DIP
switches in my SX. They were all up (on), swith #2 therefore indicating
my internal HD is using IRQ 2. I decided to call Aerocomp to find out
how to change the controller to use IRQ 5 like an IBM.
But first, I flipped DIP #2 down (off) just to see what would
happen. Well, nothing happened--the computer booted and ran fine!
Surprised the *!#@! outta me! So I left the DIP down/off.
Does this now mean I can just plug in an EGA card that uses IRQ 2?
And how come the controller still works?
Jim (boy am I confused) Unger
>>Fm: Wayne Day 76703,376
Hmmm... interesting. Don't know what kinda setup they're using on the
Aerocomp system, but my Adaptec/Seagate definitely wouldn't work that
way. Sounds like you got lucky!
I'd guess the next thing is to try the EGA card to see whether your
luck holds! <grin>
Wayne
who knows that there are SOME questions that just don't have answers!
>>Fm: Jim Unger 72747,1217
Wayne,
I spoke to a technician at WD today, who told me that since my
controller was configured to use IRQ 2, it didn't matter at all whether
IRQ 5 was enabled/disabled. He said that I was confused (mildly put).
Turning DIP #2 (IRQ 5) OFF does NOT disable 5, it merely enables an
external device to use that interrupt with no interference from another
device.
I also spoke to a tech at Aerocomp who informed me that the
WD1002-27x controller is easily changed to use IRQ 5; this I did, and
flipping DIP #2 to OFF (DISabling IRQ 5) enabled the controller to
work, using IRQ 5.
Therefore, I can now use an EGA/VGA board that uses IRQ 2. BTW, the
Everex Micro-Enhancer #657 or 659 is switchable between IRQ 2 or 3.
One major note:
The Tandy EGA board (25-3048) uses IRQ 5, so if you've got a
standard PC hard drive in your SX/TX, beware! (A std PC drive uses IRQ
5, also).
The worst part is I think I'm beginning to comprehend this modern
Sanskrit...anyway, the Everex Micro-Enhancer works on the SX or TX
because it's jumper-configurable to use IRQ 2 or 3. So no conflicts
should arise regardless of your hard drive type.
Jim
Fm: Richard W. Doig 73767,32
Jim, you are going through the same thing I did about 10
months ago. I'm a mechanical engineer, not an electrical,
but here is what I've learned in reading through the 1000SX
tech manual and experimenting.
1. An 8-bit machine also appears to have 8 input paths
available. In mechanical terms, 8 wires connecting into the
circuitry.
2. For whatever reason, these are called Interrupt Request
Lines or IRQ's for short. They are numbered 0 through 7.
3. On a 1000SX or TX , they are connected as follows:
#0 Software Timer
#1 Keyboard
#2 Connected to the bus
#3 Connected to the bus
#4 Connected to the bus
#5 Connected to the bus and internal video circuitry
#6 Connected to the bus and internal floppy drive controller
#7 Connected to the bus and internal printer port circuitry
4. What they call the bus is a series of 5 expansion slots
all connected in parallel. The six available interrupts are
actually six specific pins in each slot. So if you have a
device that can't be jumpered, you can tell what interrupt it
uses by looking at the card edge connections on the board if
need be.
5. The switch block in the SX and TX is one on-off switch
and three jumper switches. Switch #1 turns off the on-board
video circuitry. Switch #2 normally jumpers IRQ5 back into
the internal circuitry. Switch #3 jumpers IRQ6 back into the
internal circuitry. And switch #4 jumpers IRQ7 back into the
internal circuitry.
6. The net result is that switches #1, #3, and #4 act like
on-off switches for the internal video, internal floppy drive
controller, and internal printer port respectively. The
oddball is switch #2.
7. It appears that switch #2 shorts the IRQ5 connection in
the expansion slots. If you turn it to off, it severs this
short. It further appears that the system only actively
uses this connection when using the expanded video modes
available on the 1000 series.
8. Further, they have arranged it so that a hard drive
controller and the on-board video can co-exist using the same
interrupt. You actually replace the straight-through
connection in the switch with the hard drive controller.
9. Tandy's hardcard can be set for IRQ5 or IRQ2. I've used
mine set for IRQ5 for about 10 months now with no problems at
all. I just parked the drive, took it out of the case,
changed the jumpers and re-installed it. I turned switch #2
to off while I had the case open. I booted up and ran
normally. All the video modes work as well.
10. Incidentally, I thought that Tandy had dropped the list
price on the 20 meg Hardcard to $599.00 last fall. Merrymac
(GO MM) sells this same card for $459.00 the last time I
looked.
I hope this helps.
Rich
>>Fm: Tom Baden 74646,676
Jim, The way I read the Everex manual I can use any IRQ. The picture
on page 32 shows EXamples of the setup for IRQ2 and IRQ3. I just got
my TX today so I'm going to try using IRQ5 so that my Zuckerboard
hardcard can work on IRQ2 and mey Microsoft Mouse will work on IRQ3.
Wish me luck. Tom
>>Fm: Jim Unger 72747,1217
Tom,
As I understand it, you should have no problem with that
configuration. That is as long as no EXTERNAL device attempts to use
the same IRQ as some other device.
Put simply, if some un-named device (software or hardware) needs to
use IRQ 5, for example, and your HD or software requires the same
interrupt, then there's gonna be a conflict. Fortunately, in the PC
world, some of these things are standardized...a standard PC uses IRQ 5
for it's Hard Disk; most video cards use IRQ 2.
The only reason the question arose at all (for me) was that I have
one of the more PC-compatible 1000s from Tandy. The original T1k & T1ka
used IRQ 2 for hard drive; so it required a drive controller
specifically setup for that computer. If you stuck a standard PC
HD/Controller in the T1k/A, it bombed 'cause it wants IRQ 5, and
there's no way to disable IRQ 5 on the original T1ks. On the SX/TX, you
just flip motherboard switch #2 to OFF; Bingo, IRQ 5 is now disabled
for all inputs EXCEPT the external device.
Since the Everex Micro-Enhancer can be configured to IRQ2 OR IRQ 3,
it should present no insurmountable problem, regardless of hard drive
IRQ 2 or 5.
I have the jumper changes necessary to convert a WD1002-27x
controller from T1k to PC/XT compatible, if anyone's interested.
Jim
PS: Since my info is based solely on an intense study and telephone
conversations with engineers/techs, I'd welcome comments from some
of the hard/soft wizards lurking around here. Like Ray and Mike and
Horace and Wayne and -jjkd- , etc.
>> Fm: Tom Baden 74646,676
Jim, Add IRQ5 to the list, last night I booted off of my ex-T1KA
harddrive (I (IRQ2) and ran GEM using my Microsoft Mouse (IRQ3) and the
Everex driving a CGA monitor (IRQ5). No problems, except that
Starflight wouldn't use the card and monitor. I've pulled the card
until I get a Multi frequency Monitor to use the extended EGA
capabilities. All I need to do now is install the 5 1/4 drive sitting
here on the desk and wait until the local electronics store gets some
256k drams. Tom
Fm: Jim Unger 72747,1217
Wayne,
The reason your Seagate/Adaptec combo WON'T work in both DIP #2
positions is that your Adaptec controller is factory-set to use IRQ 5.
Therefore, that motherboard switch Enables/Disables the interrupt
your controller needs. Had you bought a HD/Controller combo already
configured to use IRQ 2 (Tandy 1000 standard), that IRQ 5 switch would
have no effect since your controller doesn't need or use it all.
Hey Wayne--I'm no hardware wizard, but I've made an intense effort
to get the true grit on this stuff the last couple o' days, talking to
all species of deranged hardware types. I've actually reached the stage
where I recognize a semi-literate tech on the phone who knows even less
than I do...and they usually say "Lemmee connect you with an
engineer.."....
Well, nothin's really new uder the ol' sun...I s'pose that if you
wanted to run around in one of them there horseless carriages long
about 1903, you had to be a machinist, a mechanic, or just a
masochist!!
No normal person would be seen dead in a contraption like that.
Jim