Logic Probe with 74LS257

troxel

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I am starting to understand datasheets more and more, but still not quite sure on how to test in circuit. I get when the signals at static, but when they are pulsing makes it difficult to tell if it's the correct output. Any advice?

I was testing a 257 and had Eo=L, S=H, and I1=Pulsing. Tested the output z and it was pulsing. My probe was beeping slower on the input and than on the output. Should the output be pulsing at the same speed as I1?

Hope that makes some sense.
 

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Any advice?

I take it that your probe can only test 1 input or output at a time? Really, for a chip like the LS257, which has Tri-state outputs (Low, High, and HI-Impedence), a logic probe is difficult to use.

Look into a logic clip - like the HP 10529A -- it makes things extrememly easy.

Also, I'm not sure where you got that datasheet, but the legs on a 74LS257 should be labeled:

Vcc - source voltage - 5V
GND - ground
G - enable
A / B - select
1A - input
1B - input
1Y - output
2A - input
2B - input
2Y - output
3A - input
3B - input
3Y - output
4A - input
4B - input
4Y - output
 
Really, for a chip like the LS257, which has Tri-state outputs (Low, High, and HI-Impedence), a logic probe is difficult to use.

The high-impedence (tri-state) output on an LS257 only comes into play if the enable line (pin 15) is NOT pulled low. In the OP's case, enable is low, so the tri-state situation isn't applicable. With some probes it's possible to tell when something is tri-stated, anyway.

When I use a logic probe, my practice is:
0) hook the thing up
1) touch the Vcc line to verify logic high, touch GND to verify logic low.
2) Proceed to look at each set of inputs & outputs (if they are so grouped), considering the function of the IC.

I'm typically looking for fairly obvious things like a output stuck high or low when it should be pulsing (based on the input and truth table). I've never looked as deeply as something like the speed the logic probe's LED is blinking (I'd break out the o-scope for something speed- for frequency-related). It's likely that someone more expert at logic probe use can discern more. It may also depend on the design & specs of the logic probe.
 
Also, I'm not sure where you got that datasheet, but the legs on a 74LS257 should be labeled:

Vcc - source voltage - 5V
GND - ground
G - enable
A / B - select
1A - input
1B - input
1Y - output
2A - input
2B - input
2Y - output
3A - input
3B - input
3Y - output
4A - input
4B - input
4Y - output

Same thing as original had shown...different manufacturers use different signal names but the functions are the same.

To see what is really happening - the only -real- way to see what is going on is the use a logic analyzer as Bit-slicer mentioned.
I picked up a pair of HP 1631B's (with built-in oscope) for $10. They're priceless in doing real logic debugging.
 
I guess we never really ansered your question... So, the output on a given output pin ion an LS257 is the result of the Enable pin, the select pin, the input pins, and the associated output pin. *ASSUMING* that your enable pin was low and stayed low the entire time you were testing, and *ASSUMING* that the select pin was High and stayed High the entire time you were testing, then, yes, your output should match your input -- *ASSUMING* that you read the correct input pin (the one associated with the select signal).

Now, on a real game board, these signals change constantly (possibly millions of times per second..), and they change so fast, that you really can't see the change without an oscilloscope. Sometimes, the changes are so quick, it's even hard to see them with an oscilloscope (depends on the scope..). Human ears wouldn't be able to pick up that many changes in an audible beep, and I doubt your proble could even generate that many distinct sounds per second. So, I'd say that you can't trust your ears, or your tool at this point.

Now, if you had a 74LS257 on a solderless breadboard, isolated by itself, and hooked up 5V, GND, and built a little circuit to test the chip, and supplied constant, non-changing signals to Select, Enable, and then supplied some slow, oscillating signal on the proper input pin, I would think that you could probably use your logic probe to test that chip.
 
Thanks for the help. Hypersport--that makes sense. So, my next step for me would be to compare the input and output on the scope and see if they match or assume it's fine since the output isn't floating? I don't have a logic analyzer. GPE are you telling me one of yours is for sale for $5 :). I can even pick it up and save on shipping.
 
So, my next step for me would be to compare the input and output on the scope and see if they match or assume it's fine since the output isn't floating?

Even with an oscilloscope, you can generally only check one leg at a time (some scopes have dual probes, but it's harder to keep two probes in place than one). But, the oscilloscope will show you a visual pattern, and if you see the same pattern on your input and output, then you can say things are working (so much better than audible beeps!). Again, the LS257 output pin depends on the state of several input pins. If, in the game, the select line is transitioning between High and Low, and you have two probes (1 on input A, and one on output Y), then they won't look the same, because at some point input pin B was selected, and that changes the output to match B during those intervals.

Generally (very generally...), if your outputs are changing, then the chip is working. If you see an output not changing, that is a potential for a problem. But there are lots of outputs that stay High or Low because that's how the game designer designed them, or becase a particular output pin is not used, or is tied to GND, etc. You're on the right track, if the output is correct for the given set of inputs, then the chip is probably OK. If the output doesn't match the truth table of what is supposed to be going on, then the chip is suspect.

It's really necessary to understand the schematics of the game, the logic of the chips, and how they relate to make the final determination.

So, rather than looking at the LS257, look at the chips supplying the inputs. My gut at this point (without knowing anything other than what I've heard here...) is that your select line should be toggling, at least at some point (assuming your game is running - if your game is hung, then I would expect to see nothing changing on the outputs). Maybe it is, and your probe just isn't seeing it, or maybe it's not. That's the whole point of the LS257 -- to select between lines A and B. If you are only seeing one or the other, then figure out which output pin on what chip is feeding that select line input pin, and test that chip's output to see if it is stuck high. And, if it is, look at the schematics to see if that is a design "feature", or a correct result based on the inputs for that chip. Work your way back through the chips until you find one that isn't working properly (assuming that a chip is the problem..)

Sometimes, its easier to understand what's going on with an Oscilloscope. And, at other times, it's just too damn time consuming - like when testing a flip flop. It's nearly (or is...) impossible to test a working flip-flop with an oscilloscope one leg at a time. That's were a logic comparitor (like the HP 10529A) comes in - simply install a duplicate, working chip into the comparitor, and clip onto the chip on your board, and bang - instant feedback on whether the chip is good or potentially bad (again, some chips appear bad when they aren't due to the game designers grounding output pins, or using chips in some other creative ways...). The down side to the HP10529 is that it is only capable of testing Dual In-line chips with 8 pins or less per side. If you need to test a larger chip (longer, or wider), you need something else. Also, some newer boards (like boards made in the 90's) have their chips so close together that you can't get the HP's logic clip on the chip.

There's no real replacement for an Oscilloscope though - extremely useful and versatile.

If you want to describe what game you are working on, and what symptoms you are seeing, that might help us to give better suggestions...
 
Hi;
Quick question regarding the comparator, do you clamp it onto the suspect IC while the board is energized or do you have to shut the power off each and every time you want to attach the clamp?
Thanks
Jeff
 
Hi;
Quick question regarding the comparator, do you clamp it onto the suspect IC while the board is energized or do you have to shut the power off each and every time you want to attach the clamp?
Thanks
Jeff

You clamp it onto the running board for most chips. For things like flip-flops, you should clamp with power off, then turn the power on so that the chips sychronize equally during power on / boot up initialization.

Also, touching some chips with the comparitor can crash the game. For these sensitive areas, clamp with power off, the turn the power on.

Lastly, some chips (like those leading to the RGB/S signals) on the edge connector are sensitive as well, and clamping the comparitor on those may distort your display, or kill the video to where you need to reboot the board again. Even if you clamp fist with power off, the display may not work with the compaitor hooked up. Good thing here is, if you have a good display, the chips are more than likely working properly, and so no need to clamp on those chips.
 
Excellent, so the general procedure is "live" with an exception or two.
I was concerned about damaging the board set but now I see that the worst case would just require a re-boot.
Thank you for the info.
Jeff
 
Excellent, so the general procedure is "live" with an exception or two.
I was concerned about damaging the board set but now I see that the worst case would just require a re-boot.
Thank you for the info.
Jeff

Well... that depends on how sloppy you are with the clip... if you clamp in between legs instead of right on the legs, you could theoretically damage a chip . Or worse, if you accidentally clamp on a high voltage chip when you meant to clamp on some other chip, you might run into a few problems. Don't learn on your favorite board I guess is what I am saying... But there really isn't too much to fear - you generally use the clip on a non-working board anyway...
 
Ref the Comparator, it's very easy to use if you have the multi IC test board included. The manual isn't very clear, I could make a short video showing you how to use it. You clamp the comparator to the suspect IC then insert a known good IC in the comparator test board. You set all the dip switches on the test board to closed and then open the dip switches for any output pins on the IC and power up the board...if you see any lights on then 9 times out of 10 the IC is bad. an IC ref book is a must in order to set the dip switches on the test IC quickly.

http://cgi.ebay.com/Hewlett-Packard...691573978?pt=BI_Analyzers&hash=item19c5bb60da

here you go.
 
Thanks for all the information. A lot of material to sort through. I will say that Womble and ajcrm125 are two people that have documented their repairs quite well. Appreciate all the information with this so far.
 
Thanks for the help. Hypersport--that makes sense. So, my next step for me would be to compare the input and output on the scope and see if they match or assume it's fine since the output isn't floating? I don't have a logic analyzer. GPE are you telling me one of yours is for sale for $5 :). I can even pick it up and save on shipping.

You're not gonna like this...
I had -three- logic analyzers. Two HP 1631B's and a Gould K105D. The Gould worked great but weighed a trillion lbs...donated it to the junk man about a month ago.

As far as selling one of the 1631B's... one works fairly well, the other is for parts. The one that works fairly well has a tube that's getting dimmer. The parts machine has a bad pod #0 (main pod) interface... probably fixable if I had the time. One of these days, I'll have a good, bright and everything works logic analyzer (like the Gould was).

If you or anybody else ever do go shopping for used logic analyzer -- ALWAYS make sure it comes with cables, pods AND leads. They're useless without any of these... and nearly impossible to find on their own. Seems that most of the time, logic analyzers on ebay are shown without these or not enough info to verify if they are. Best to ask first if not specified.

Ed
 
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