Lab 4, Part II: Frequently Asked Questions

Exercise 3

I'm not sure I understand what the circuit is supposed to do for Exercise 3, so I have no idea how to set it up....

The answer to what the heck this thing is supposed to do is contained in Step 4: "The purpose of this circuit is to..."

What that is basically saying is that you want to create a circuit that gives a true output when the 2 switches on top are set the same way as the two switches on the bottom.

So, if the two switches on top (A1 and A0) are set to, say, 01 (that is to say the A1 is set to 0 and A0 is set to 1), and the two switches on the bottom (B1 and B0) are also set to 01 (B1 is set to 0 and B0 is set to 1), then you should get a 1 for your output.

However, if the two switches on top are set to 01 while the two switches on the bottom are set to, say, 10 (or 11, or 00), then the output will be 0.

Okay, I think I understand what the circuit's supposed to do, but I have no idea how to hook it up...

There are only 4 possible ways the circuit can be connected. In necessary, you can try them all until you arrive at the correct result.

You know that the "Mystery Gate" is either an AND or an OR, but you also need to make sure you connect the switches to the XORs correctly -- either the unconnected top switch (A0) can go to the upper XOR, and the unconnected bottom switch (B1) to the lower XOR, or that top switch can cross down to the lower XOR and the bottom switch can cross up to the upper XOR.

Exercise 4

How the heck do I do this?

To understand how the TruthTable gates work, you need to work through the Activity section material on pp. 48-49 of your lab manual. You'll be making 2 TT circuits, each with 3 inputs and one output.

For the 1st, the 3 inputs will be the A, B, and Carry-In columns from the Truth Table on p. 110 of your text, and the output will be the Sum column.

The 2nd will have the same input columns as the first, but the output will be the Carry-Out column.

The three switches will connect to each truthtable, and each truthtable will be connected to its own output box.

I can't figure out how to change the truth table once I have set it down.

Assuming the applet is working correctly, you should be able to follow the procedure as described in the Activity section for this lab:

"Click once on the truth table (after making sure that all the buttons on the left are off, i.e. do not have asterisks in front of their names.) This makes the truth table the top gate. Now click on EditTT. A tiny green window pops up and lets you edit this box's truth table."

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