Sys 80 Non CPU boot/IRQ issue

[MartyJ]

If the tests are done correctly then it is a good chance that U2 or U3 is bad.  Also try it with the game prom removed. and test the IRQ then to ensure this is not dragging it down.

I will do this now and report back.  I think between me learning how the logic probe works and my nice new shiny DMM I believe I've done everything right as per Clay's guide. 

I've emailed Ed again - asking very very nicely if he has a QuickScan tucked away somewhere (shop listed sold out) of his QuickScan's....Just in case I get a HH or another Sys80 type game...Plus probably handy for someone locally to have one in case others get into trouble!

[MartyJ]

Yep, still high with PROM1 removed.

[Strangeways]

If the tests are done correctly then it is a good chance that U2 or U3 is bad.  Also try it with the game prom removed. and test the IRQ then to ensure this is not dragging it down.

I will do this now and report back.  I think between me learning how the logic probe works and my nice new shiny DMM I believe I've done everything right as per Clay's guide. 

I've emailed Ed again - asking very very nicely if he has a QuickScan tucked away somewhere (shop listed sold out) of his QuickScan's....Just in case I get a HH or another Sys80 type game...Plus probably handy for someone locally to have one in case others get into trouble!

I was going to buy one just for that very reason. I have several system80s in the pipeline and I wanted to have a Quickscan handy for others that may run into problems in the future.

Let me know how you go - I might want one as well Marty.

[MartyJ]

If the tests are done correctly then it is a good chance that U2 or U3 is bad.  Also try it with the game prom removed. and test the IRQ then to ensure this is not dragging it down.

I will do this now and report back.  I think between me learning how the logic probe works and my nice new shiny DMM I believe I've done everything right as per Clay's guide. 

I've emailed Ed again - asking very very nicely if he has a QuickScan tucked away somewhere (shop listed sold out) of his QuickScan's....Just in case I get a HH or another Sys80 type game...Plus probably handy for someone locally to have one in case others get into trouble!

I was going to buy one just for that very reason. I have several system80s in the pipeline and I wanted to have a Quickscan handy for others that may run into problems in the future.

Let me know how you go - I might want one as well Marty.

Ed emailed me back.  No luck.  He is well and truely out of stock with the boards.  He is hoping to start production again but no time frame unfortunately.

[ddstoys]

Hey Marty My new CPU arrived today and all went well around $350 all up so if you get stuck its an easy option

[MartyJ]

That will be an option but I'm not going to be beaten by the Sys80...Its a challenge!  But because of Dave coming to the rescue with the driver board, worst case I can do it.

I'm waiting on a U2/U3 adaptor and I've ordered enough spares for the displays in case segments are out etc from GPE...

I desoldered them both cleanly tonight...I may redo U6 as well, just for completness.

IF from my reading and the magic of the logic probe, all things are pointing to those being the culprets

[ddstoys]

Yeah i would have stuck at fixing this one myself if it was mine but being for someone else and having other better games waiting for this one to go i just bit the bullet *%  BUt good luck with yours

[MartyJ]

U6 replaced, PROM1 replaced, U2/U3 replaced.

IRQ still high.   
 

[MartyJ]

5101 replaced as well.

Now I'm really stuck!

[Marty Machine]

Not sure on the polarity of the IRQ for that cpu chip, but typically IRQ's are meant to be high, and only pulled low when controlled from the other chips onboard. (of course your cpu may be the opposite of everything else).

Regarding your AC measurments on pins 15/16 (0v) did you try measuring with both MM probes on 15 AND 16?
It looked like you might have measured from pin15-to-ground, and  pin16-to-ground, which would probably measure 0v, misleading you.

Any chance of running your display board down to Mark's to plug into a known-working pin to be SURE your displays are ok?
I also find it odd that only 60V is mentioned on your displays, typically Vacuum displays require pos & neg volts, typically +/-50v upto +/-100v etc....

just my 2c 
MM.


 

[MartyJ]

Not sure on the polarity of the IRQ for that cpu chip, but typically IRQ's are meant to be high, and only pulled low when controlled from the other chips onboard. (of course your cpu may be the opposite of everything else).

Regarding your AC measurments on pins 15/16 (0v) did you try measuring with both MM probes on 15 AND 16?
It looked like you might have measured from pin15-to-ground, and  pin16-to-ground, which would probably measure 0v, misleading you.

Any chance of running your display board down to Mark's to plug into a known-working pin to be SURE your displays are ok?
I also find it odd that only 60V is mentioned on your displays, typically Vacuum displays require pos & neg volts, typically +/-50v upto +/-100v etc....

just my 2c 
MM.
 

Thanks MM,

I think I've got the display issue sorted.  From everywhere I've read pin 4 of U1 (CPU) the IRQ should stay high for the first five seconds, then drop to pulse (at 3.5v I think).
Mine is stuck at 5v and not pulsing - hence the board not booting.

I have just about given up on this.  Its nearly got me beat.  I cannot work out what else can be causing issues with the IRQ.

[Marty Machine]

Don't let it beat you 

IF you look at the schematic, what other chips/pins does the IRQ signal go to?

Obviously a pin is holding it 'stuck' high, your logic probe should find this.
If chips are socketed (or you socket them) you can bend 1 pin out, so it doesn't plug into the socket, and although the chip will be working you can now measure the pin itself to see if it's outgoing IRQ control to the cpu is 'stuck' or not.

You would test chip-by-chip until lifting one of the pins out of socket brings the IRQ on your cpu back to life....

Failing the pin-bend method, perhaps just lift out all chips that control the IRQ cpu pin, then replace the chips one at a time, you should see the irq 'blinking' (as you said) until plugging 1 chip back in kills it again...

Another option is that all the IRQ pins are ok, and in fact a shorted irq track on the board etc.

If you point me to a schematic/link i can help you test/prove things too.
MM.

[Strangeways]

Marty - I'll have a read of the schematic and see what I can think of.

I have an untested SYS80 board here (from my Force II project), that you may borrow if it makes things easier.

[MartyJ]

New CPU in, couple of this resolved but still IRQ high.

From reading the schematic, IRQ begins at pin 4 U1 (6502) goes via R1 & R2 to pin 16 TC1 then to pin 25 U4, U5 & U6.

Is there anything prior  to U1 which has anything to do with IRQ?

[Marty Machine]

New CPU in, couple of this resolved but still IRQ high.
From reading the schematic, IRQ begins at pin 4 U1 (6502) goes via R1 & R2 to pin 16 TC1 then to pin 25 U4, U5 & U6. Is there anything prior  to U1 which has anything to do with IRQ?

TO help you follow the 'flow' in the diagram, the IRQ signal is going INTO pin 4 on the the 6502cpu.
( maybe this will help you follow signals with your probe/MMeter).

IRQ (Interupt ReQuest) is a signal that comes from all the other chips into the main cpu.
IRQ will pull low to interupt the main cpu(6502) to process information in the chip that interupted the cpu..

It's a way for an external chip to say to the cpu "hey, i have fresh data for you, stop what your doing and process my data"

MM.