Is a logic comparitor a big fat waste of money?

jehuie

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I was looking at these as a possible tool to help track down the watchdog problem I'm having on my asteroids. But I've read mixed reviews. Are they useful? Or would I be better off investing in a scope and learning how to use it?

That seems like it would be a huge investment not only in money but in time to learn.
 
I have one that I picked up dirt cheap and I haven't actually used it (yet) to repair anything. I suppose it could come in handy for certain chips, but it won't even work on the larger form factor chips, so it's definitely not a comprehensive solution.

Your best bet for bang for your buck in the repair department is a good set of schematics (freely available online) and a logic probe with audible (beep) feedback ($20 on Ebay).

A scope is great to have, and few would argue that money spent on a scope is money wasted. Mine doubles as a test bench vector display, which is a good thing.

An eprom burner is another great repair tool--it will allow you to check ROMs for their integrity as well as program replacements.

I'm a big fan of my Fluke 9010 troubleshooter; great for checking RAM and ROM from the CPU perspective, and dealing with address lines. With the integrated data probe it's a pretty versatile repair tool.

The real gurus favor logic analyzers, but I'm not allowed to buy one yet as I am still a Padawan learner.
 
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The comparator is only useful if you have a good idea which chip is bad, and have another to compare it to. If it's cheap enough, I suppose it's useful, but it looks kind of limited in application to me (note: I've NEVER used one, so it could be I just don't know how useful it is).

I'd go for a scope long or a logic probe first, because those allow you to poke around the system as it runs. If price is an object, definitely go logic probe first (they are lots cheaper than a scope). If money isn't an object, you'll want a scope and a logic analyzer.

Of course, I'm assuming here that you already have a good DMM. If you don't, that needs to be number 1 on your list.
 
I've got an old HP model I picked up off eBay cheap, as mentioned though, you've got to have a working example of the chip under test, and a pinout so you can set the lines correctly.

When you think about it, swapping out adaptors, chips, and setting a dozen or more dips each time you want to test a chip is a big hassle. On the other hand, when it works, it's like magic and of course there are those times when you have multiples of the same chip to test and things go a little quicker.

If you're considering one, be sure to get the adaptors with it, or better yet, a ZIF socket multi dip switch version, instead of the dedicated etched versions which only do a single chip per adaptor.
 
Thanks. Good feedback. I'll probably skip this tool based on what I'm hearing. It just sounded interesting when I was reading about one. I've got a DMM and a logic probe. I'm not great at reading schematics yet so it's hard for me to track down problems like the one I'm having now. I thought it might be a way to randomly check chips.

You know, it seems like in this day and age someone could program a device to emulate just about every known chip in the world so that you could use it as a comparitor without having the original chip on hand. Perhaps there isn't that much demand for that kind of device though since traditional troubleshooting techniques are probably a better approach.
 
I have a buddy with an HP model comparator. We were thinking about trying to use it in troublehooting a Quadrapong board since it's all discreet logic 74xxx TTL chips. That seems like a good way to use it in that case.
 
I've got an HP10529A, brand new never used with all the fixin's (or at least most of them! :)) $45 shipped in the US. I can take pics.
 
I've got an HP10529A, brand new never used with all the fixin's (or at least most of them! :)) $45 shipped in the US. I can take pics.

Hmmmm. That's veeeery tempting. My problem is that then I would need to start collecting spares of every chip I might want to test right?
 
I don't have one, but I was told ( and tell me if this is wrong ) that the same chip from 2 different manufacturers can register differently in some cases. I would say that the comparator is far from a magic wand, but could be usefull in some instances.
 
I have a buddy with an HP model comparator. We were thinking about trying to use it in troublehooting a Quadrapong board since it's all discreet logic 74xxx TTL chips. That seems like a good way to use it in that case.

For what its worth, I'm an extreme newb when it comes to board repair, and I have both a logic probe and a decent 100mhz o-scope. Between the two, the $5 flea market logic probe has proven to be the most useful device on TTL stuff. I was able to get my board 50% working last night using nothing more than a multimeter and a logic probe.
 
For what its worth, I'm an extreme newb when it comes to board repair, and I have both a logic probe and a decent 100mhz o-scope. Between the two, the $5 flea market logic probe has proven to be the most useful device on TTL stuff. I was able to get my board 50% working last night using nothing more than a multimeter and a logic probe.
Yeah I've had a logic probe for a while and it's helped a little before. I guess what confuses me is how you know what each leg of a chip should be reading on the probe. Is it the same no matter what pcb you find it on? Would a datasheet tell you?
 
Datasheets would tell you. Actually at a local hamfest (electronics flea market) I happened to find a TTL chip catalog for a buck, and wow did it make it soooo much easier to look each chip up! I can post the title/info later but I think it was just a national semiconductor book from 1986.

Something like this (and equally as beat up), and I just took a wild "hey this looks like something that might be useful" guess and man did I hit the nail on the head with that purchase!

http://cgi.ebay.com/NATIONAL-SEMICO...lectrical_Equipment_Tools&hash=item518dac4dfd
 
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For what its worth, I'm an extreme newb when it comes to board repair, and I have both a logic probe and a decent 100mhz o-scope. Between the two, the $5 flea market logic probe has proven to be the most useful device on TTL stuff. I was able to get my board 50% working last night using nothing more than a multimeter and a logic probe.

How did you get started learning to trace the problem with a logic probe? I've done some where I had good instructions. But in the case of this Asteroids, I don't even know where to begin. Other than verifying the watchdog has kicked in.
 
Datasheets would tell you. Actually at a local hamfest (electronics flea market) I happened to find a TTL chip catalog for a buck, and wow did it make it soooo much easier to look each chip up! I can post the title/info later but I think it was just a national semiconductor book from 1986.

Something like this (and equally as beat up), and I just took a wild "hey this looks like something that might be useful" guess and man did I hit the nail on the head with that purchase!

http://cgi.ebay.com/NATIONAL-SEMICO...lectrical_Equipment_Tools&hash=item518dac4dfd

That's great....I'll have to find one!
 
Now that I'm home - thats almost exactly the book I have. In fact, here's the 1984 version...

http://cgi.ebay.com/National-LOGIC-...lectrical_Equipment_Tools&hash=item1c115c32f1

Did some searching and supposedly at one point they were offering the books free or pretty cheap, and I also found this..

http://www.national.com/JS/searchDocument.do

Which is pretty good for quickly looking up a datasheet if you keep a laptop nearby, but honestly the book is nice because it can be right there.

More datasheet links:

http://www.libraries.uc.edu/libraries/engr/databooks/index.html

As for what to do, or where to start, I think thats half the battle. Get the manual/service manual/schematic for the board, and go from there. For a watchdogging processor - do the watchdog bypass and see what happens. If the game seems to work, its possible that something in the actual watchdog circuit is bad. If it still doesn't work, then work your way back from the watchdog. For example, on a 6502 processor, pin 40 is the watchdog. Its an active-low watchdog, meaning, when that pin goes LOW, its watchdogging. When its high, its working. From there, trace that back to what does the watchdog, and figure out why its going low.

Now that said, I spent a lot of time doing exactly that on my video pinball board (again I'm a newb to this stuff), and what that really provided was a good lesson on using a logic probe and how a watchdog circuit works. In the end, I realized I had no video, started at the end (video) and worked my way back and quickly found an actual issue. I had no video, checked the chip that provided the video, found one of the signals its supposed to get was stuck high, traced that back to a different chip, found the chip was working except for that pin, replaced it, and now I have a picture again. (although still not 100% fixed). Just an example but sometimes you need an example to get a grasp of this stuff. I wish I could just watch someone who fixes these things for a living troubleshoot a board one day, just to see their train of thought.
 
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Thanks for the links, orion. For some reason I didn't think to check my copy of Fromm's Big Blue Book. It has some good info on TTL logic too so hopefully alll this will get me started in the right direction.
 
When I bypassed the watchdog, the screen went blank and the flashing start-buttons went dark. The spot-killer came on very faintly. Not sure what this means to me though. Also, when it's watchdogging, I get a pulsing on pin 40.

. For a watchdogging processor - do the watchdog bypass and see what happens. If the game seems to work, its possible that something in the actual watchdog circuit is bad. If it still doesn't work, then work your way back from the watchdog. For example, on a 6502 processor, pin 40 is the watchdog. Its an active-low watchdog, meaning, when that pin goes LOW, its watchdogging. When its high, its working. From there, trace that back to what does the watchdog, and figure out why its going low.
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I wish I could just watch someone who fixes these things for a living troubleshoot a board one day, just to see their train of thought.

So do I. Anyone know of any videos on YouTube or elsewhere showing someone troubleshoot a board? If any of the regualr KLOV board fixers were to post a video of them working on a board, I'm sure a lot of us newbs would be most appreciative.
 
I would volunteer to give my board over for the cause. You know, you could fix it and we could video it. :) Yeah, I knew it was a longshot.... :)
 
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