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Three-Input DTL NAND Gate

Introduction

One of the limitations of RTL circuits was that while NOR gates are easy to construct, it is difficult to obtain a NAND function in a single gate. Theoretically we could stack multiple transistors in series, but this leads to a range of problems.

With DTL, however, we can simply add more input diodes to the inverter circuit. This effectively combines a Diode Logic AND gate with an inverter circuit. The use of the inverter re-amplifies the signal and thus overcomes the limitations we saw in the plain DL AND gate.

In this experiment, we will extend our DTL inverter according to this idea, and see how well it works.



Schematic Diagram

The DTL NAND gate combines the DTL inverter with a simple Diode Logic (DL) AND gate. Thus, any number of inputs can be added simply by adding input diodes to the circuit. The problem of signal degradation caused by Diode Logic is overcome by the transistor, which amplifies the signal while inverting it. This means DTL gates can be cascaded to any required extent, without losing the digital signal.

In addition, the use of diodes in this configuration permits the construction of NAND gates; something that was not practical with RTL because there was no practical way to allow any single logic 0 input to override multiple logic 1 inputs. Therefore, DTL offers more possible configurations as well as better performance than RTL.



Parts List

To construct and test the three-input DTL NAND gate circuit on your breadboard, you will need the DTL inverter you constructed in the previous experiment, plus the following experimental parts:



Constructing the Circuit

You should still have the DTL inverter circuit in place from the previous experiment. If you did not perform this experiment, or if you removed the parts, go back now and reassemble the inverter circuit. You will expand on it for this experiment.



Circuit Assembly

Start assembly procedure

Beginning the Assembly

Before you begin installing new components, verify that your breadboard socket still has the DTL inverter circuit in place from the previous experiment, as shown to the right. If not, go back and assemble that circuit now. If you did not perform the experiment previously, you should perform it now.

Click on the `Start' button below to begin installing the additional components for this experiment.

1N914 Signal Diode

Locate a 1N914 silicon signal diode and form its leads to a spacing of 0.4". If you performed the Diode Logic experiments, you'll have two diodes left over with this lead spacing, and can use one of them. Install this diode in the location shown in the assembly diagram to the right. Be careful to observe the orientation of the diode.

Click on the image of the diode you just installed to continue the construction of your experimental circuit.

1N914 Signal Diode

Locate a second 1N914 silicon signal diode and form its leads to a spacing of 0.5". Install this diode as shown in the assembly diagram. Again, be careful to observe the orientation of the diode.

This diode will go right next to the DIP switch. That's OK; everything will still fit.

As before, click on the image of the diode you just installed to continue.

3" Orange Jumper

If you still have a 3" orange jumper left over from earlier experiments, use it here. If not, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end. Connect one end of this jumper to logic switch S1 and the other end to the point shown in the assembly diagram.

Click on the image of this jumper to continue.

3" Orange Jumper

Either locate or construct another 3" orange jumper, and connect one end to logic switch S2. Connect the other end to the point shown in the figure to the right.

Again, click on the image of this jumper to continue.

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 switches S0, S1, and S2 all to logic 0. Observe the output state of your experimental circuit, and record this logic state on the top row of the table to the right.

Continue by setting the three input switches to each of the eight possible input combinations. For each input combination, note the state of the output as indicated by L0. Record your results in the table to the right.

When you have recorded your results for all input combinations, look over your results and decide what kind of logical function is actually performed by this circuit. Is it really a NAND gate?

When you have made this determination, turn off the power to your experimental circuit and compare your results with the discussion below.

Inputs Output
S2 S1 S0 L0
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1


Discussion

You should have found that this gate produces a logic 1 output as long as any of the inputs is at logic 0. Only when all inputs are at logic 1 will the transistor be allowed to turn on and produce a logic 0 output. This is an inverted AND, or NAND function, so this circuit does indeed behave as a 3-input NAND gate. The number of inputs can be increased to any desired extent by simply adding more input diodes, and the circuit will still exhibit NAND behavior.

When you have completed this experiment, make sure power is turned off, but leave your experimental circuit intact; you will modify it for your next experiment.


Prev: DTL Inverter Next: Two-Input DTL NOR Gate

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