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Piggy backing logic chips on top of each other

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May 11, 2015
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For troubleshooting logic circuits technicians will place a good same part number TTL or CMOS logic chips on top of a suspected bad logic IC chip. Can this piggy backing technique work with any logic chips, or will the logic chips be fighting logic states?

Not to make any disparaging comments about any technicians on this forum (that do this), but I don't think a technician that trouble shoots a design like that is qualified to be a technician.

Suppose the problem is a stuck output due to a fault in the chip or maybe a ground short on the pin. Now you are just having another part get potentially damaged trying to drive that ground to a high logic level. Besides that the only way to make sure you have a good connection is to solder the piggy back part onto the other part (note this only works for DIP and maybe PLCC, LLC, and gull wing type packages, i.e. those with leads, forget about BGAs).

The best technicians I've worked with, would ohm everything out, and use a scope probe to look at the signals coming out of the chip, if they are sure it's the part that is faulty and not some external short, then they would remove the part and check the board again, before putting a new part on the board, if that fixes the problem they would check the removed part for failure and if it's still a broken part send it over to the group that does failure analysis.

So the piggy backing technique wouldn't work when troubleshooting TTL and CMOS logic boards?

Any reasons why it won't work?

It might work for a small subset of problems, but it is a poor way of debugging anything. It is probably right up there with shotgunning a design (randomly replacing parts until the board works).

Do you think it will damage the piggyback IC chip also? or the logic states of the piggyback IC and the bad IC on the bottom with be fighting logic states?

If there is a short to ground or short to VCC , the piggyback IC will get damaged right?

If there is a short to ground or short to VCC , the piggyback IC will get damaged right?
I already said that was a possibility in post #2.

Smells like Walters again right?

In the '60ies I worked with a pretty big office computer that had a built in printer. Many of them jammed when the paper was pulled out before the page was printed. I fixed them by piggy-backing another DTL logic IC on top of an existing DTL logic IC and made one trace cut and one jumper was added. Basically I added some logic but the added IC did not fight with the existing IC. The management of the company I worked for compensated me for my quick fix that worked perfectly.

You piggybacked another IC to ADD and modify the circuit

I'm talking about piggybacking that is a troubleshooting technique to isolate which logic chip is the problem on a TTL or CMOS logic board. To to just solder an IC ontop of another IC to modify a circuit that is different subject matter. I have done that many times because there is no room on a pcb board. companies do that all the time, don't feel special!!!

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