DK NMI interrupt Bad

RyDawg

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Well, I got a DK2 for christmas.

I went to install it today - and it gives the error NMI interrupt bad. The board was working before, but I honestly haven't powered it up for quite a while.

Is this just a Z80 chip issue? Will a different Z80 fix it, or is there some kind of damage to my board? I know nothing about NMI, but some guidance would help.

thanks!
 
It's probably got nothing to do with the Z80. Like most games, the NMI's triggered by ther vertical sync/blank, and cleared in the interrupt handler...

If the MMI was actually bad, the game wouldn't have played before... chances are the kit just isn't seated well in the socket, or you bent a pin (17) on the Z80 when moving it to the daughterboard...
 
Checked it this morning.

Replaced with another Z80 - no change.

The kit is seated properly, I checked all pins. I took the kit out and went to power the board up, and it gives me a screen just like the one that says 'bad 5a' on this site:

http://www.brasington.org/arcade/tech/dk/

So, that site is saying bad 5a, but the DK2 kit is saying bad NMI interrupt. Could it be an issue with the pots on the board (V/H sync) ? If I recall, 5A is replaced with the kit, so it shouldn't be a problem with 5A.
 
Ohm out from the output of the 7474 that generates the NMI to pin 17 of the CPU.

That should pulse low every 1/60 of a second... but in your case it should either not be gong low at all (missing vblank / bad flop) or staying low (not getting sensed & reset by the CPU, bad NMI reset logic / bad flop).

I'd have to hack through Braze's crappy code to find out exactly how it's "detecting" a bad NMI, and my decryption of his ROMset is on an old computer in storage.
 
Did you put the original 5A back in, not the one from the kit, and try it ?

The game freezing like that also happens if the NMI ( non-maskable-interrupt ) of the z80 isn't being triggered. So it's not necessarilty a bad 5A.

Just to be sure check the z80 socket make sure the socket pin isnt' bent in so that it's not touching pin 17 (NMI) of the z80.

If you've got a logic probe, do the following.

1. check pin 17 of the z80, see if it's pulsing which I think it's safe to assume it's probably not ;)
2. Check pin 5 of IC 8F (74LS74) see if it's pulsing. If it is you'll need to check continuity betweek pin 5 and pin 17 of the z80.
could be a broken pin in the socket (??)
3. check pin 3 of IC 8F see if it's pulsing, it should be.
if it's not pulsing check pin 6 of IC 6F, that should be pulsing, if not then 6F is likely bad and should be replaced.
4. check pin 4 of IC 8F on the CPU board, when you get to the screen showing high score and the game is frozen that pin should be pulsing.
if pin 4 is stuck low and never goes high that points towards a problem at IC 5H, LS259.
5. Check pin 1 of IC 8F make sure it's high.
6. Check pin 2 of IC 8F make sure it's low.

If all that's checks out good and no z80 NMI on pin 17 then 8F needs to be replaced.
 
I put the original 5A and Z80CPU back in - and when I power the board, it comes up as I listed on the Braze site.

I only have a basic DMM to check problems with.
I checked continuity betweek pin 5 of 8F and pin 17 of the z80. It checks out good.

I'm assuming then the problem is likely to be 8F or 6F? I realize there are other things that could go wrong, but there are good chances.

Apart from that - are the numbers the only difference in IC? I have several 'junk' boards I can pull from, and I see some 74LS74's, but they have some different manufactuer/letters/etc on them.

The DK board has: SN74LS74AN from El salvador 8131A.
I have an elevator actoin junk board that has a MB74LS74 (8152 M47) and MB74LS74 (8203 305). Just curious. I have no issues ordering IC's, It's more curiosity and using parts I have if I can.

Maybe I'll ask for a logic probe for X-mas.

Thanks so much for all your help.
 
You have no concept of irony do you? :rolleyes:

When you get any sort of technical competence and learn about programming, perhaps your opinion of my code (which you probably haven't reviewed and can't understand) might have some merit.

It's pretty obvious looking at the D2K code, which parts were written by Braze, and lifted from the original DDK / DK score save kits, and which were written by Jeff more recently.
 
When you get any sort of technical competence and learn about programming, perhaps your opinion of my code (which you probably haven't reviewed and can't understand) might have some merit.

It's pretty obvious looking at the D2K code, which parts were written by Braze, and lifted from the original DDK / DK score save kits, and which were written by Jeff more recently.

WHOOOOOOOOOOSHHH....

That's the sound of Irony going over your head once again....
 
I'm assuming then the problem is likely to be 8F or 6F? I realize there are other things that could go wrong, but there are good chances.

Yup, that's where I'd make a start.

Apart from that - are the numbers the only difference in IC? I have several 'junk' boards I can pull from, and I see some 74LS74's, but they have some different manufactuer/letters/etc on them.

You shouild have no problem swapping the parts from different manufactures, as long as it's a 74LS74 you will be fine. Normally the first couple of digits are a manufacture code.
eg. SN = Texas instruments MB = fujitsu.

Maybe I'll ask for a logic probe for X-mas.

They're a really useful tool for basic troubleshooting, you can determine a lot of problems just with just a probe. Everyone should have one ;)

- James
 
Well, thanks for all the help.

I swapped out some chips one at a time. The 8f wasn't it, the 6F wasn't it. I finally swapped out the 5H, and that wasn't it either, but now I think it's worse. It still shows the interrupt as bad - but the diagnostic screen won't stay on. It automatically tries to run the game.

I doubt I'll look at this much further until way after the holidays, and by that time I might just get a different DK board.

Thanks a ton for the help though.
 
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