Digital Troubleshooting and testing

[walters]

What are some basic Digital Troubleshooting techiques?

When digital troubleshooting using the logic probe how do you know if the digital componenc is working right?

How do you signal trace a digital circuit or component to know if its working right to track down the componect thats not working ?

For digital troubleshooting how can i use the oscilloscope to test the digital components and circuit

a.) Test and display the timing ?
b.) See the On and off timing?
c.) What else can i test for on the oscilloscope?

 

[Thunderchild]

just test the inputs and outputs with the logical proble and see if they corespond to the truth table of that particular gate as a starter !

 

[walters]

For Testing Gates in a Circuit do i inject a HIGH or LOW signal to the input of the GATE and use the logic probe on the output of the gate ?
with the truth table like to me ?

 

[Thunderchild]

yes carry out all the possible combinations so for a two input gate that is 00, 01, 10, 11 and see if the out put is correct for each state. just make sure u r injecting the apropriate signal for that type of gate and if u r injecting it externally and not testing the signal already present then i advise u to remove the chip from the circuit (these should be pluged into sockets so that should not be a problem) maybe using a bread board or standalone socket with wires to it will help u in the testing.

 

[walters]

THanks for the information

How can i do in-circuit testing for digital components?

Gates troubleshooting in-circuit testing?
Clock troubleshooting in-circuit testing?
registers troubleshooting in circuit testing?

How do i test and troubleshoot decorders or encoders?

 

[Thunderchild]

u can test te circuits own signals or inject a signal and hope for the best there generally should not be a problem to remove the chip is just a precaution - u did use sockets in your project DIDN'T you ?

 

[walters]

Well im talking more about a product for company's that make a board
or circuit thats digital and knowing how to troubleshoot it and how to start
testing digital componects that my problem of knowing digital troubleshooting skills what should i know? and how to do this?

 

[Thunderchild]

well as for the professional way i have no idea and not having an osciliscope because of its cost i would resort to a simple homemade probe but the method is up to u perhaps u should try and find a good book on the subject if u want to go into it on that large a scale as i suppose u would have to know the various signal levels that each family of chips use.

 

[walters]

Yes whats the professional way for digital troubleshooting ?

 

[Thunderchild]

there is no specific method i suppose it depends on the situation what u prefer and what u have at hand i mean a multimeter can be used if nothing else is at hand. i think the most professional instrument available i cannot remember the name of but is is like an osciliscope and can veiw in some up to like 16 signal simutainesly. basical i suppose what makes a professional is knowing how to deal with each situation. some companies even design for their products custom circuit testers that have a board with dozens of probes sicking up and the faulty circuit is placed on top and the whole board is analysed at once but this is a computerised machine and is specific for each board and is only feasable on a large scale. it just depends on what u r working on and what u have available.

 

[walters]

How would u test digital components like registers,encoders,decorders, decimal to hexadecimal, gates?

 

[Thunderchild]

with the digital oscilisope thing and a signal injector. connect the inputs of the instrument to the outputs in quetion and probe with the injector and verify the results. if u r dealing with encoders and decoders then i think u have like 8 inputs ? for the different addresses so perhaps something like a word generator is necedary and can be wasily built it is just as multy out put signal injector

 

[walters]

But how do i know if the encorders and decorders are working right?
I got the oscilloscope on inputs and the outputs

Yes the encoders have 8 inputs and the decoders have 8 outputs

How do i test the Encoder and decoder Addresses? with a probe or oscilloscope

what do Encoder and decoder Addresses look like on a oscilloscope ?

Are Encoder and decoder Addresses voltages? or a waveform?

 

[Thunderchild]

digital 1 is normally VCC and digital 0 0volts so onthe oscilicope singal tracer or what ever u use u will see 0 or VCC (or there abouts as theory is never practice even in digital stuff) so say u have the encoder. this has for example 3 address inputs 2 to the power of 3 is eight therefor u can have 8 inputs being encoded into 1 output when u input address 0 (000) input 0 is sent to the output when u input ADD 1 (001) in 1 goes to output and so on until input 7 and add 7 (111) so u have to produce all the 8 addresses (000 - 111) and verify that the signals uinput to the relevant input are coming from the output. basically just get the blasted truth table and create urself all the states on it. u also have to understand what the chip does proporly to know what is going on and be able to understand the data sheet as it may make a very breif explanation of the chip and it's truth table.

 

[walters]

Is there a easier way to do this without truth tables testing digital componects checking the inputs and outputs the truth tables can take along time to check all the High and low states don't you think thats really hard to track a digital componect down thats not working right i would have a bunch of truth tables then?

 

[Thunderchild]

well to be thorough u need to test every combination (hence that complicated custom equipment i mentioned earlier) but if u know the area of trouble or are able to work it out then it could be simpler. it is not that comliccated u know ? once u have atached the proble to go through all the combinations is childs play just check that u put an 0 and a 1 signal through to make sure that what goes in comes out that just means an on off switch on the input probe or your puting it on and off. or if the chips are socketed and are not that meany (or u have pleanty of cash) just change all the chips. or get replacements say one per type and change them to make sure they work

 

[walters]

So should i start with the Clock pulse first to see if the Timer circuit it sending out a clock pulse?

What are the basic steps from Left to right to start from in digital signal tracking or tracing ?

I have to Check and Test each chips Pin Voltages , Clock Pins, Iinput and out states high or low , what else should i test for or know?

 

[Thunderchild]

start from what apears to be the begining that could be the input of the whole circuit u may want to ensure fist that all the chips r receiving a clock signal if they r supposed to as there could be some clock buffereing circuitry or not gates that may cause a general fault as no clock signal mean no go for anything

 

[walters]

Thanks for helping me out on this

So when troubleshooting or signal tracking digital circuits and components you have to have alot of truth tables ?

I have a logic probe and that takes forever to test ever chip pin to with the truth table to see if its working right

What are some basic digital signs or problems that digital circuit get or have to be more experienced ?

 

[Thunderchild]

the truth table will tell u in ste most convenient way what a circuit does i mean if u were a near geneise u could look at a truth table and know from that what the chip is not/and/nor/en/decoder ecc. there r no common faults as far as i nkow with digital stuff as one chip is worth another i suppose but a hint: what is all this digital circuitry controlling? if it is like transistors that perhaps control relays then check allthat first. i know this is the end of the circuit but the power stuff is more prone to go than the digital stuff and is less time comsuming.
(god what question is that going to lead to i suppose - perhaps u had better tell me what this thing is and does)