replacing ALS08 with LS08

parism

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Hi guys, I have an OutRun with a barking watchdog that is driving me nuts to diagnose. In the watchdog circuit there is an ALS08 IC that my logic comparator says is OK, compared to a standard LS08.

My question is whether the IC being ALS compared to LS may throw my comparator off.

Can I replace an ALS with an LS? My take is that SEGA placed an ALS there for a reason…

Thoughts on this?

Thanks.

p
 
see: https://en.wikipedia.org/wiki/7400_series

74LS – Low-power Schottky. Implemented using the same technology as 74S but with reduced power consumption and switching speed. Typical 10 ns gate delay, a remarkable (for the time) 2 mW dissipation, 4.75–5.25 V.

74ALS – Advanced low-power Schottky. Same technology as 74AS but with the speed/power tradeoff of the 74LS. 4 ns, 1.2 mW, 4.5–5.5 V.
 
Thanks, from what I have read I can replace an LS with an ALS but I am not sure if the reverse is also true...
 
You'd need to look at the rest of the circuitry to see if it's a suitable substitute. The LS is not a guaranteed replacement for an ALS, but it many cases it could be. But if Sega did specifically use ALS, then LS probably isn't sufficient though.

I'd say you're probably "barking" up the wrong tree though (get it? ;) ). When a board is watchdogging, it's rarely the watchdog circuit. Try disabling the watchdog and see if the game runs properly... it probably won't.

Regarding the logic comparator... any of the series' should be fine since IIRC (at least the HP one) only flags a mismatch when they're different for >50ns. The propagation delay on any of those parts will be <50ns. That also means that if you've got a part that's causing a problem because it's slow, your logic comparator won't help.

DogP
 
Thanks, that was great information. The watchdog circuit (as I see it) is attached. My focus is to eliminate all possible culprits.

The /WDC line going into LS192 is actually HIGH which makes me believe the culprits may be either the MB3771 or the ALS08 that feeds the /CRST and /RESET lines in the PCB.

Good news is that the CLK to the 68K CPUs is fine and at the necessary frequency (10MHz). However, the /HALT and /RESET lines and constantly flip-flopping.

Already found 1 sub-program EPROM that was a dud but replacing it did not fix the issue. Piggybacking with good SRAM for each 68K CPU does not do much although occasionally may stop the watchdog BUT without booting the game.

The weird thing is that the some of the audio section is actually working, looking at the activity on the EPROMS and associated SRAM and the custom audio IC.

This is all to strange and this is why I am taking things one a time...
 

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Hi guys, I have an OutRun with a barking watchdog that is driving me nuts to diagnose. In the watchdog circuit there is an ALS08 IC that my logic comparator says is OK, compared to a standard LS08.

My question is whether the IC being ALS compared to LS may throw my comparator off.

Can I replace an ALS with an LS? My take is that SEGA placed an ALS there for a reason…

Thoughts on this?

Thanks.

p

When using your logic comparator, you should always use a reference IC of same family of the UUT.So, you canìt use an LS to test an ALS.It can work too but specs are different (fanout, power consumption and propagation delay time of gate).
But replacing on arcade boards an ALS with LS works most of times unless the IC is involved in some critical circuit (like timing)
 
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Thanks Caius, unfortunately I did not have an ALS to use with my comparator. However, I ended up extracting the 08 and confirming it good in my eprom burner. In any case, after repairing a burnt trace, the pcb boots to no video with the /reset and /halt lines on both 68k cpus flip flopping. Disabling the watchdog by applying a 5v on the /crst after boot up result in video with some artifacts. This is better than before so progress is made. To be continued....
 
Yeah, I do have them but I don't see any floating/stuck signals on them when the watchdog is bypassed. I have tried to piggyback them but I know this does not always work if the ram is failing (but not failed). I will look into this a little more and report back. I know the issue is in the CPU board, that is half the battle (i guess)...

What I don't know is if the audio section does not work whether it will cause the watchdog to be triggered.
 
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What I don't know is if the audio section does not work whether it will cause the watchdog to be triggered.

No, they two separate systems although in some boards a not running sound system will halt the main CPU.But I think it's not the case of Out Run.
 
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