Defender Address line 6

saltbreez

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Hey Guys,

I've got a Defender MPU board giving me fits. Address line 6 goes high after about 20 seconds. I am not certian if this is just a symptom or the primary cause of this boards troubles.

I have socketed and thus rulled out the MPU buffer [74ls367] at 2F, I can also rule out the RAM as well as the ROM board. This still leave a LOT of chips touching A6.

Any ideas what might be causing this? Any suggestions to facilitate debugging? I would hate to shotgun all chips touching A6.

Thanks,
Saltbreez
 
Hey Guys,

I've got a Defender MPU board giving me fits. Address line 6 goes high after about 20 seconds. I am not certian if this is just a symptom or the primary cause of this boards troubles.

I have socketed and thus rulled out the MPU buffer [74ls367] at 2F, I can also rule out the RAM as well as the ROM board. This still leave a LOT of chips touching A6.

Any ideas what might be causing this? Any suggestions to facilitate debugging? I would hate to shotgun all chips touching A6.

Thanks,
Saltbreez

Nobody? Does this board get shelved for a stuck address line?

Saltbreez
 
This still leave a LOT of chips touching A6.
Not really. If A6 is going high out of the 6809 and you have replaced 2F, then outside of some kind of short related to board heating, I don't see how any other chip could cause this. Since A6 is an output only, 2F provides effective isolation between it and the rest of the PCB. You could put a dead short on 2F pin 3 and it should not impact the A6 input.

Obvious question, but when A6 goes high, I assume that the relevent 6809 pins like reset, halt, etc are ok? Do any of the other address pins get "stuck" either low or high? The only other thing I can think of is the 6809 code stuck in a loop which is both located at an address with A6 high and which only references data with A6 high.

Have you swapped with another MPU board? Are you certain that is where the problem is?
 
Hey Guys,

I've got a Defender MPU board giving me fits. Address line 6 goes high after about 20 seconds. I am not certian if this is just a symptom or the primary cause of this boards troubles.

Does it get into the RAM test at least or just sit dead ?

- James
 
...

Obvious question, but when A6 goes high, I assume that the relevent 6809 pins like reset, halt, etc are ok? Do any of the other address pins get "stuck" either low or high? The only other thing I can think of is the 6809 code stuck in a loop which is both located at an address with A6 high and which only references data with A6 high.

Have you swapped with another MPU board? Are you certain that is where the problem is?

I did check the reset and halt lines, reset pules correctly when the reset is pushed, halt was correct to my understanding at the time [although I do not remember if that was high or low].

I use this rig to test several Defender boards, this is the only one doing this.

Thanks,
Saltbreez
 
Just sits dead, garbage lines on screen [NOT jailbars].

Thanks,
Saltbreez

Soon as the CPU starts up it jumps to the RAM test, so I'd say you're not even seeing the CPU write to the video RAM then high A6 is just a symptom. If it only sticks high after 20 seconds then the 6809's being doing something all that time.

Check the E & Q clock inputs to the 6809. This is where a dual channel scope comes in useful as you can easily see them on one trace, the E clock should go high 1/2 way thought the high-period of the Q clock, and go low 1/2 way through the low period of Q.

Check the CPU databus on both sides of the 74245 at 1K, you should see all the datalines pulsing.
Also check/swap 3K the address bus PROM just in case it's bad, at which point the CPU would have some difficulty writing to RAM. Also check the outputs on 4K to see if they ever pulse low, showing the CPU writing to the video RAM.

- James
 
Hey Guys,

I've got a Defender MPU board giving me fits. Address line 6 goes high after about 20 seconds. I am not certian if this is just a symptom or the primary cause of this boards troubles.

Have you considered that the game may just be running code and in a loop for which A6 is always high?

I call red herring.
 
Have you considered that the game may just be running code and in a loop for which A6 is always high?

I call red herring.

From Wikipedia, the free encyclopedia:

" A red herring is an idiom referring to a device which intends to divert the audience from the truth or an item of significance. "

I think you are confused with your usage of idoms, what were you trying to convey?.

Yes, I have considered the possibility that the "game may just be running code and in a loop for which A6 is always high?", however, this provides no insights to getting this board to a functional state.

Saltbreez http://en.wikipedia.org/wiki/Red_herring_(idiom)#cite_note-0
 
Soon as the CPU starts up it jumps to the RAM test, so I'd say you're not even seeing the CPU write to the video RAM then high A6 is just a symptom. If it only sticks high after 20 seconds then the 6809's being doing something all that time.

Check the E & Q clock inputs to the 6809. This is where a dual channel scope comes in useful as you can easily see them on one trace, the E clock should go high 1/2 way thought the high-period of the Q clock, and go low 1/2 way through the low period of Q.

Check the CPU databus on both sides of the 74245 at 1K, you should see all the datalines pulsing.
Also check/swap 3K the address bus PROM just in case it's bad, at which point the CPU would have some difficulty writing to RAM. Also check the outputs on 4K to see if they ever pulse low, showing the CPU writing to the video RAM.

- James


Thanks, James,

I will check it out and report back.

Saltbreez
 
From Wikipedia, the free encyclopedia:

" A red herring is an idiom referring to a device which intends to divert the audience from the truth or an item of significance. "

I think you are confused with your usage of idoms, what were you trying to convey?.

Yes, I have considered the possibility that the "game may just be running code and in a loop for which A6 is always high?", however, this provides no insights to getting this board to a functional state.

Saltbreez http://en.wikipedia.org/wiki/Red_herring_(idiom)#cite_note-0

Gee, do you think I might have been referring to the fact that A6 going high -- a symptom on which you appear to be fixated -- is diverting your attention from the actual problem with the board?

...which is most likely an issue with ROM 1/2 or the interface between them and the CPU.
 
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