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Two-Input TTL NOR Gate


TTL integrated circuits provide multiple inputs to NAND gates by designing transistors with multiple emitters on the chip. Unfortunately, we can't very well simulate that on a breadboard socket. What we can do, however, is use the same technique that we tried in both RTL and DTL, to form a NOR gate. This requires separate inverter transistors just as it did with the other logic families.

In this experiment, we'll construct a TTL NOR circuit and test its operation.

Schematic diagram of a TTL NOR gate.

Schematic Diagram

The figure to the right shows the schematic diagram of a TTL NOR gate. As you may recall from earlier experiments, it is substantially the same as the RTL and DTL NOR gates we have already explored. The only difference is that this time we are using the TTL input circuitry.

Parts List

To construct and test the TTL inverter circuit on your breadboard, you will need the TTL inverter circuit from the previous experiment, plus the following experimental parts:

Constructing the Circuit

You should still have your TTL inverter circuit in place on your breadboard socket. If not, or if you did not perform the TTL inverter experiment, go back and do that now. This circuit is an extension of the TTL inverter, and requires that you have the inverter in place as a starting point.

Circuit Assembly

Start assembly procedure

Beginning the Assembly

Make sure the right hand side of your breadboard socket is clear of all experimental components. You'll be re-using a number of components here, but with different placement and interconnections.

Click on the `Start' button below to begin the assembly of your experimental circuit.

0.3" Black Jumper

You should have several 0.3" black jumpers left over from previous experiments. If not, prepare a new 0.3" black jumper. Install this jumper in the location shown in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue on to the next assembly step.

0.2" Bare Jumper

It's not generally worthwhile to fuss with insulation for a jumper that's less than 0.3" long. Therefore, simply form a length of bare hookup wire (or a clipped component lead) into a jumper with 0.2" spacing. Install this jumper on your breadboard socket as shown to the right.

Again, click on the image of the jumper you just installed to continue.

4.7K, ¼-Watt Resistor

You should have a 4.7K, ¼-watt resistor (yellow-violet-red) left over from your DTL experiments. If not, locate a new 4.7K, ¼-watt resistor and form its leads to a spacing of 0.5". Install this resistor as indicated in the assembly diagram.

Click on the image of the component you just installed to continue.

2N4124 NPN Transistor

You should have several 2N4124 (or 2N3904 or similar) NPN silicon transistors left over from previous experiments. If not, locate one now and form its leads to a spacing of 0.1" so it will fit readily on your breadboard socket. Install this transistor in the location indicated in the assembly diagram. Be sure to observe the required orientation of the transistor.

Click on the image of the transistor you just installed to continue on to the next assembly step.

2N4124 NPN transistor

Locate a second 2N4124 transistor and, if necessary, form its leads as before. Then, install it in the location indicated to the right. As before, observe the required orientation.

Again, click on the image of the transistor you just installed to go on.

3" Orange Jumper

Locate one of the 3" orange jumpers you have used in most of your experiments. If you don't have one, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end. Connect one end of this jumper to S0, and the other end to the point shown in the assembly diagram.

Click on the image of the jumper you just installed to continue on to the next assembly step.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure

Performing the Experiment

Turn on power to your experimental circuit, and set both S0 and S1 to produce a logic 0. Note the resulting state of your experimental circuit as indicated by L0. Record this value on the first line of the table to the right.

Now move S0 to the logic 1 position and record that result in the table. Continue with the remaining possible combinations of S0 and S1, noting the state of L0 in each case.

Turn off the power to your experimental circuit when you have finished recording its behavior. Then, look over your results. What function does this circuit perform? Compare your results with the discussion below.

Inputs Output
S1 S0 L0
0 0
0 1
1 0
1 1


With this circuit, you found (as you probably expected) that L0 indicated that the gate would output a logic 1 only when both inputs were at logic 0. The moment either input became a logic 1, the output dropped to logic 0. This is the correct behavior for a NOR gate, so this circuit does indeed perform the function it was intended to perform.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Remove all experimental components from your breadboard socket and put them aside for use in other experiments.

This concludes our set of experiments on basic digital logic families. Soon to come will be experiments that will use these basic circuits to perform a range of useful and interesting tasks.

Prev: TTL Inverter Next: Multivibrators

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