# How to determine IC function

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?

• 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. – Keelan Jan 28 '15 at 23:25
• We haven't done those in class, but I'll look into it. Thanks! – Lefty Jan 30 '15 at 1:37