basic digital troubleshooting

[danny davis]

I don't really know how to troubleshoot for digital circuits , but what are some basic digital measurements and test to do for digital chips?


1.) First measure the supply Voltage on the chip to make sure it's getting voltage or there is a short from VCC to ground that's why the chip is not working

How do I know a digital chip is or digital circuit is working?

What are some basic rule of thumbs to test for digital chips and digital circuits please?

 

[Harald Kapp]

For simple digital ICs (40xx, 74xx and the like) and static logic:
Look at the datasheet of the circuit. It will tell you what the inputs and outputs are and what the logic between input and output is. Record the logic levels at the inputs and outputs of the chip. Compare with the function as described in the datasheet.

For simple digital ICs and dynamic logic (Latches, Flipflops etc.):
As before, but you need to take into account the timing. If your timing signals are very slow or can be triggered manually, record the signal states before and after a clock edge. Compare with the datasheet.

For both purposes a logic probe (http://en.wikipedia.org/wiki/Logic_probe) is a useful tool.

For more complex ICs and/or higher clock rates a logic analyzer is a helpful (but expensive) tool. Depending on the application it may be useful to build a dedicated debug tool. For example debugging a counter can be aided by connecting a display to the counter's outputs so you can watch the numbers counting up (or down) in the right sequence.

Harald

 

[danny davis]

Thanks for the info.

What are some basic "test procedures" to test for digital electronics in general for electronic chips in the 70's and 80's?

What are some basic things to look for to know if the chip is working or not working?

 

[Harald Kapp]

Basically you stimulate the circuit with defined signals and check the reaction. Apart from checking the power supply a logic probe and a logic analyzer were known even back in the 70's and 80's

Harald

 

[danny davis]

They had logic analyzers in the 70's?

I have a logic probe, but what tests /checks can I do with it?

At work they have logic analyzers but I don't know what kind tests or checks to do?

If I have the datesheets for each digital chip, how can I use the logic probe or logic analyzers to do what tests or checks?

 

[Harald Kapp]

As I said: watch inputs and outputs and compare the measured function with the expected (datasheet) function.
Read the manual of a logic analyzer to learn what to do with it and how.

You don't always need fancy tools to debug a circuit - if you know what you do. Sometimes a simple LED indicator put in the right place can be a great help. And sometimes even a logic analyzer will be no help because your problem may be analog (yes, even digital logic is based on analog principles).

And yes, they had it in the 70's. An excerpt from http://www.hpmemory.org/wb_pages/wall_b_page_12.htm:

Over a brief period from 1969 through 1973, Logic Probes, Clips, and Analyzers appeared - tools that became known as data domain instrumentation. By 1977, when HP fully arrived in computing, the Logic lines were solidly established.

Here's some more info:
http://en.wikipedia.org/wiki/Logic_analyzer
http://www.metrictest.com/resource_center/pdfs/logic_understanding_trigger.pdf

Harald

 

[danny davis]

inputs and outputs and compare the measured function with the expected (datasheet) function.

Yes i get that, but what i don't get is do i use a function generator and set it to a square waveform?

What Do I use to inject the input signals ? a function generator?

Wouldn't I need a function generator with 10 outputs to connect to the inputs of the chips, plus a clock signal ? to monitor the outputs to compare with datasheets?

 

[Harald Kapp]

How to stimulate your circuit depends on the type of circuit, the problem etc. You need to generate stimuli that fit the problem at hand.

There is no general solution. For a counter a function or square wave generator may be suffcient. For a microcontroller a test setup using switches, pushbuttons et. may be required.

 

[Rleo6965]

It's still the best to understand the components and circuit first and then monitor the inputs and resulting outputs. Oscilloscope was the best equipment for me. Because you can see the waveform. ie. if digital signal does not reach the required digital threshold or output causing the digital circuit malfunction.

 

[Harald Kapp]

Because you can see the waveform. ie. if digital signal does not reach the required digital threshold or output causing the digital circuit malfunction.

That's what I meant by

because your problem may be analog (yes, even digital logic is based on analog principles)

Harald

 

[danny davis]

how do you troubleshoot each stage of a digital circuit?

Because troubleshooting analog circuit you use a sinewave function generator on the input and monitor each stages output until you find no signal

When troubleshooting digital circuits the stage or chip can be good but it not working because of other chips around the circuit

So that's why I think you have to "inject" signals on the inputs , clock, reset to make sure the outputs monitor measure right with the datasheets?

It's hard to measure the inputs of the circuit before the chip you're testing because the stage before the digital chip you're testing might need multiple chips to work properly so it does that function

It takes multiple chips for each stage to give you an output to measure

All the chips can be good when tested out of circuit but in circuit they all test bad is because each chip is relying on certain inputs or outputs of different chips. That's why digital troubleshooting for me is more complex because each digital chip inputs and outputs are connect scattered to make another digital chip trigger its inputs and outputs to function to trigger another digital chip

So the troubleshooting is not testing inputs to outputs from left to right like audio analog circuits

My problem of digital troubleshooting is how to check if a chip is good when it relying on other chips in the circuit before it or surround it to be TRIGGERED or other chips to trigger other chips in other stages or surround circuits?

because if the digital chip has multiple inputs , those input signals come from other digital chips from other stages or surround stages.

Do you de-solder each chip one by one and check the inputs and outputs compared to the datasheets

 

[Harald Kapp]

So the troubleshooting is not testing inputs to outputs from left to right like audio analog circuits

Why not?

You have multiple components, you have feedback eztc. just like in an analog circuit. And just as a sine wave generator is nit suitable for testing every analog circuit so is a square wave generator not suitable for testing every digital circuit.

As I said before: you have to look at the specifc function of the circuit (regardless whether analog or digital) and orient your test stzrategy accordingly. I don't know of any cookbook to that end.

By the way: there are twotypes of testing:
1) Find out whether a newly developed circuir operates as intended. This is often called debugging.
2) Find the fault in a previously functinal circuit.
The strategies for testing are different because in case 1 you can't be sure tht the circuit works at all or whether maybe a design flaw makes it non-functional.
In case 2 you know that the circuit has worked before and you look for the defect.
But again: the test strategy depends on the circuit.

Harald

 

[Rleo6965]

The secret of successful troubleshooting of analog or digital circuits was first understand all components and how this components arranged in circuit to form a functional block. Once you understand the functional block. This will help you analyse which block have failed to result certain defect. ie. you don't have to lift all ic in the circuit board to be able to find the defective ic or component.

"Know your enemy before you conquer". You will have higher chance of winning.

 

[CocaCola]

"Know your enemy before you conquer". You will have higher chance of winning.

That is the best answer... You develop a battery of test for each design you will be testing, and with every successful fix you can start taking notes on how that fault was detected, and what the readings at point(s) were... After several repairs you will have a list of test to run on that device that will detect your known faults... The more you repair the larger the list will be and the more comprehensive, pretty soon you will be diagnosing repairs on that design without even a thought process...

 

[danny davis]

If you have the Datasheets for each IC chip

What are some In circuit test that you can do? since you have the datasheets

What in circuit tests can I do having the datasheets

---------- Post added at 08:07 ---------- Previous post was at 07:58 ----------

The way I do it is when i have the datasheets for a IC chip
I put the oscilloscope probe on the output pins to monitor the output
With the inputs I use a square wave function generator set to +5 volts
I use a Seconded Function generator set at square waveform for the clock input
And I use a pushbutton for the Reset input

I will try each input and output compared with the Datasheets with that IC chip

Most of the Time the IC chips will check out GOOD, but when logic is turned on it circuit doesn't work

Because I test each IC chip like in the circuit or stage to give you a result like turn on a motor or light,

But how do you find out which chip is bad if they all test good like the way i do it?

 

[Harald Kapp]

If the chip "tests good" but doesn't work in the circuit, either the circuit is faulty or your test does not cover the required fucntion of the chip correctly.
As I said before: a function generator may be suitable for testing one type of chip but may be entirely useless for another type of chip.

You need to design your test procedure depending on the type of chip and possibly on the application, too.

And don't forget that any testing is mostly done not in real time but much slower. So even if the chip seems to be O.K. when testing at low frequencies, It may not work in the application due to timing problems. You'd need a logic analyzer to find that issue..

Harald

 

[Rleo6965]

If you can post diagram of circuit with motor problem. We might give you idea how troubleshoot that circuit. Then later apply those technique in future repair.

 

[danny davis]

timing problems. You'd need a logic analyzer

What do timing problem cause or how do you know your circuit has a timing problem?
some basic symtoms of Logic timing problems

The circuit would do what please if it had a logic timing problem?

 

[Rleo6965]

When your computer hangs up.

Then your solution. Power off and restart computer or press the Reset button.
Reset button will initialize all ic to its default or correct state. Depending on the design of circuit.

 

[Harald Kapp]

What do timing problem cause or how do you know your circuit has a timing problem?
some basic symtoms of Logic timing problems

The circuit would do what please if it had a logic timing problem?

The circuit can do anything from not working at all to working perfectly under certain conditions (like e.g. only with higher supply voltage than normal - compare with PC overclocking techniques).

To be honest, I don't understand what exactly you want from us:
You've been given loads of general information. as much as one can expect without a detailed description of the specific problem at hand. If you don't know what timing problems are: Do you have a general understanding at all of how digital logic circuits work? As in any troubleshooting (analog or digital): You first have to know what to expect from your circuit, then compare this with the actual behaviour of the circuit. From the diffference, whatever that may be, you work forward to isolating the fault.

Harald