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Multi point continuity tester circuit?

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Advanced Member level 3
Sep 7, 2009
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Lancashire UK.
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Hello can someone advise on how I might go about making a circuit for a multi point continuity tester?

I have a very large PCB to reverse engineer that I intend to strip all the components off and want to figure out the connections from pin to pin using a continuity test (it will take a long time).

There are over 100 16 pin IC's and 50 20 pin IC's.

so to speed this up I am proposing to use 14/20 pin probes that fits onto the PTH IC pins ICA/ICB which then check:
ICA pin 1 to ICB pin 1
ICA pin 1 to ICB pin 2
ICA pin 1 to ICB pin 3
Until it gets to pin 16 or 20

Then goes to>
ICA pin 2 to ICB pin 1

And so on.

When a short is detected an LED lights showing the pin no for probe A and another LED for the pin for probe B.
This way I can then write down the connected pins - the test should continue no further until I press a button to continue.

Once all pins are finished I will move probe B onto another IC socket to test that pair.

Can anyone suggest a circuit for this or perhaps an existing tester that might do this?
Or perhaps another method to test the pin to pin connections?

I am thinking of an IC for each probe, outputting to 16/20 led's that steps through each pin - I just cannot remember which sort to use and would appreciate a direction to look in?

Thank you.

- - - Updated - - -

Scribbling on some paper, I am thinking of 3 x 4017 decade counters for probe A that is fed by a clock signal at say 1 hz.

This would put a voltage on each pin in turn - 1-16 (I would want to switch to to also be 1-20).
Then another IC connected to each pin for probe B that has outputs that illuminates an led for each pin if there is a voltage on the input.
At the same time, if there is an input I want an output from each led (perhaps a dioded one) that stops the clock until I press a button to continue (to give me time to note the led no's).

But what IC would be at the receiving end?

You have 2600 pins to test!!! I think that means you have, literally, millions and millions of tests to perform. For example, you have to test first pin 1 of your first IC against 2599 pins, then pin2 against 2598 pins, pin3 against 2597, etc. Just testing the first IC will require tens of thousands of tests!! (My lack of statistical abilities is evident here.)

I would just quit my job and open a flower shop.

Or find a schematic for the board in question.

You could build a tester, but that sounds like a lot of work for just one board. If you expect to sit there and look at LEDs blinking on and off, the chance for making a mistake is pretty high.

Yes, actually I have 4000 pins to test.

This is just to reduce some of the time by doing these IC's quicker.

Probe IC1 to IC2 then IC1 to IC3 and so on.
I would expect to be able to probe an IC in a few seconds unless there is a connection which takes a few seconds each one to write down.

Mistakes are expected, it will be done more than once to be sure.

But there is little other choice.

My thoughts :- First check out the Vcc/0V pins - these tend to be constant on N pin packages. So once these have been tested, then there is no reason to check pins 7 and 14 (for 14pin TTL) .Once this has been sorted, then the pin 7 & 14 can be removed from the test schedule. Now moving over to the other pins. It is important that the "moving" probe is modified so it just makes a contact with all the pins, no sliding locking mechanism to actuate! First test would be interpin connections on IC, scan them at 1mS/pin, beep on short, stops test, displays input pin number (green ) and output pin number (red ) , LEDs must be spread out a bit 10mm + between AND labelled with number. Press button - test proceeds (to a count of 24 - for BIG ICs). At end all LEDs flash. Move test probe to next IC . It would be a nice idea to load the data into RAM as you go, just to double check at a later date!

Yes that's kind of what I am planning, however they are not all the same sort of IC so gnd/vcc cannot be guaranteed at the same points.

Although I may be able to add some detection between gnd/vcc to light an extra couple of leds to show shorts to them (seperate connections for probe A required).

I'd love to do this with a Raspberry pi so it can record it all in a text file - but thats beyond me as it will need programming.

Strip the board of the components, scan it, use some EDA software that can import bitmap image as a template and you are done.

That works great for multilayer boards. No, wait, no it doesn't.

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