Watchdog inhibit

MrGorf

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I'm working on a time pilot 84 board and the watchdog keeps tripping. Is there any way to inhibit it so I can see whats not starting?
 
I'm working on a time pilot 84 board and the watchdog keeps tripping. Is there any way to inhibit it so I can see whats not starting?

usually I just tie the reset line on the cpu high...1k resistor to 5 volts > reset pin on cpu.

I use an old ink pin for this put a sewing needle in the tip with resistor to wire with clip to +5 for changing logic states.
 
I'm working on a time pilot 84 board and the watchdog keeps tripping. Is there any way to inhibit it so I can see whats not starting?

Not sure inhibiting the WD will help you debug why its not booting. If you inhibit it the game will most likely run until it hits a point and then locks up.. but then what? How do you diagnose what caused the lock up?
 
It would only help if its the actual watchdog circuit causing the problem. Not very likely but it doesnt hurt to rule it out.
 
I figure if both CPU's feed into the watchdog, and I can keep the watchdog from tripping, I might be able to see which CPU isn't operating correctly.
 
Have you downloaded and reviewed the schems?

http://arcarc.xmission.com/PDF_Arcade_Manuals_and_Schematics/Time_Pilot_84_Schematics.pdf

The reset circuit (both power-on and watchdog) is shown in the top-right of the 1st sheet. Its output is the "/RES" line.

Looks like there is a split-pad (look at the line feeding into pin 10 of the LS00 @ 8G) that may function as inhibit; you'd have to spend a few minutes with a databook to follow the logic. Also note that the watchdog section of the circuit appears to be reset by the "/MAFR" and "/SAFR" lines. Each of these appears to originate from the address decoding from each CPU; /MAFR from 3A (left side of schem), and /SAFR from 13E (top middle). A little logic probe or o-scope work on these signals may help determine which CPU is failing to do its thing.
 
Looks like there is a split-pad (look at the line feeding into pin 10 of the LS00 @ 8G) that may function as inhibit.

That did the trick. Bridging the pad keeps the output of the LS00 from toggeling and shuts down the watchdog.

When I power up, it starts the self test and does one of three things. I either get a bad RAM message, good RAM but bad ROM, or it goes into attract mode (though with some blocks or other artifacts on the screen).

I did notice that on the LS04 at 13F, pin 11 had a good signal bit pin 10 was dead. It looks like this enables ROMS 12A-15A. I replaced the LS04 and fixed that, but I've noticed if I press on the chip at 11A, the screen changes. The socket for that chip looks pretty crappy, so I think my next step is to swap that out.

Also, it looks like someone either changed the sockets for the RAM at 4F and 5F or resoldered it, but they did a poor job, so I'll swap those sockets out and clean that up.
 
As soon as I see previous repair work done on a non-working board, I zero in on that portion and verify that the connections are sound.

Bill
 
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