TTL debugging techniques

It happens all the time, even to experienced professionals. You wire up a digital logic circuit, but it doesn’t work the way you expect. Here are some of the most common reasons why a digital circuit might not work, along with tips on how to diagnose the problem.

Check the chips’ power…

  • Is the proper power supply (typically 5VDC) connected to each chip?
  • Is Ground connected to each chip? Is it at 0VDC?
  • Are power and ground connected to the correct pins?
  • Is the power reasonably clean? (Check it with an oscilloscope.)

Check for common wiring mistakes…

  • Are any pins on the chip left unconnected? If so, should they be?
  • Are any of the chip pins bent underneath it, instead of making contact?
  • Are enable pins connected and tied to the correct value?
  • Are any outputs shorted together? (This is a Bad Thing™)
  • Are any outputs tied to power or ground?

At this point, take a logic probe and start tracing signals on chips that seem to be misbehaving. Follow the “contract model” of diagnosis: You provide power, correct signals, correct power, and an output of sufficiently high impedance, and the chip performs the correct function. If it doesn’t, and your conditions are OK, replace it!

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