I have a lab assignment coming up in which my instructor is tasking us to determine the function - essentially construct a truth table - of several unknown integrated circuits of varying difficulty. Specifically, the mystery chips will be confined to TTL SSI chips, only combinational logic.

First I suppose one would have to determine the inputs from outputs. For TTL gates, unconnected inputs will register about 1.7V. How could the outputs be determined? Wouldn't they fall within the range <.5V or >3.5V?

Once the inputs are determined, next I could experiment with different inputs (0's and/or 1's) and observe the output voltage, or even connected a LED to the output to see the consequence. Besides that, I'm not sure what I could do to figure out the function of the chip.

One of the chips will be broken - we aren't required to identify the defect, we are only required to report the intended function of the chip. Also, one of the chips will have "open" collector outputs. Would the aforementioned system still hold for these scenarios?

  • \$\begingroup\$ I'm on my phone now, so I can't write an answer easily, but if you are required to give a boolean algebraic formula for the outputs, you could have a look at Karnaugh diagrams. \$\endgroup\$ – Keelan Jan 28 '15 at 23:25
  • \$\begingroup\$ We haven't done those in class, but I'll look into it. Thanks! \$\endgroup\$ – Lefty Jan 30 '15 at 1:37

You are on the right track. You can usually determine which pins are input by measuring the voltage. Do note that an output pin that is logic HI can have a voltage as low as 2.4V and still be within spec.

After you have identified which pins might be input, then use a 1K resistor to ground to determine which pins are inputs. Touching a 1K resistor from ground to an input pin will result in the pin voltage dropping significantly, whereas an output pin won't drop significantly. Mark all those input pins on your drawing.

Now use a 1K resistor tied to +5V and touch any pins that showing as 0V. If the voltage rises slightly, this is an output pin that is LO. If the voltage rises to 5V, this may be an open-collector pin that is HI. Note that it might also be a "No Connect" pin.

Now that you have identified probable input and output pins, connect LEDs from the 5V rail through current limit resistors to all of the pins that might be outputs. Then start applying stimulus to the input pins, first singly, then in combination. Use a table to record any changes that you see. The reason that you are driving your LEDs from 5V instead of Ground is so that you can see changes on pins that are open-collector.

Create a binary table with enough entries to cover all of your input pins so that you can be sure to test all possibilities.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.