Digital Logic Laboratory Exercises
Wiring & Checking A Logic Chip - The 7400 Quad NAND
Experiments With NANDs

Using Logic Gates

If you want to use gates, you will need to learn something about their physical characteristics.  In this section we'll walk you through wiring a simple gate circuit using one specific integrated circuit (IC) the 7400 chip.  It's a good introduction to some of the more complex logic chips that you'll probably be using later.

Lab Objectives

You need to know what you should get from this laboratoroy lesson.  Here are the lesson objectives.

Given a circuit that requires the use of standard 74xx logic gates,
Be able to connect power to the gate,
Be able to check functions of the gates on a chip,
Be able to wire gates together to produce more complex logic circuits taking into account limitations on the number of inputs to gates on a chip.

Wiring A 7400 Chip

Here's a picture of the 7400 chip in a circuit board.  This chip is actually an N74LS00P.  The LS tells you that it is a low power Schottky chip.  Every manufacturer will embed the 7400 or 74LS00 in other part numbers.

Notice that this chip has fourteen pins.

If you want to use an IC chip, then you will always need to know the pinout.  That's electrical engineering lingo for describing the way the pins are connected to the internal circuitry of the chip.  You need to know where the power supply is connected and where the gate inputs and outputs are connected.  Here's the pinout for a 7400 chip.

The first step in wiring the 7400 is to connect the positive power supply.  Use a five volt (5v) power supply and don't turn it on yet.  Connect a lead to pin 14 as shown below, and connect the other end of that lead to a 5v supply.  Keep the power supply turned off until you have everything connected.  Here's what that looks like when the positive supply voltage to the chip is wired.

The next step in wiring the 7400 is to connect the ground connection.  Connect a lead to pin 7 as shown below, and connect the other end of that lead to ground.

Notice the pattern to this connection.  The power to this digital logic chip goes to the corners.  Remember, power to the corners for logic chips.

Now you can connect the two inputs to one of the gates on the chip.  You're going to put 5v on either of these inputs for a 1 and ground the input for a 0.  There are two wires in the picture below that connect to pins 1 and 2 on the chip.  Those pins are the inputs for one of the NAND gates on the chip.

Now you can connect the output of the gate.  You will need to connect this output to something like a voltmeter or an oscilloscope so that you can measure and observe the output of the gate.  (And the voltmeter or oscilloscope will also have to be connected to the ground.  You will measure output voltage with respect to ground.)  The output will be near 5v when the output is a 1 and near 0v when the output is a 0.

Actually, you can often connect LEDs to give a visual indication of a 1 (LED lighted) or a 0 (LED dark).  Here some LEDs are shown, together with 1kW current limiting resistors.  If you connect LED indicators to your circuit remember that an LED is not the same in both directions, and you have to get the correct end connected to the resistor.  The other end of each LED is connected to ground (or just "grounded").  Here's the circuit to show the output of a NAND gate:

When the output of the gate is a 1, the output voltage will be five (5) volts.  Current will flow through the series combination of the resistor and the LED, so the LED will light.  When the output of the gate is a 0, the output voltage will be zero (0) volts and the LED will not be lit.  Thus, the LED lights up when the output is a 1, and doesn't light when the output is a 0.  You can use this indication scheme to show the status for any signal.  (It doesn't have to be the output of a gate.)

Q1

In the picture above, is the power turned on for the chip power supply?

Here you can "test" the circuit.  For the input combinations, click the hotword and observe what the situation should be.  (THIS PART UNDER CONSTRUCTION)

Inputs

1 2

0 0

0 1

1 0

1 1

At this point you're ready to fire up your circuit.  You can turn on the power supply and test a gate on the 7400 chip.  After you've done the experiment and recorded your observations you're ready to go on.

Some NAND Gate Experiments

1. The pin-out for the 7400 is shown to the right.  Using NANDs from the 7400:

• A. Design, construct and test an inverter.
• B. Design, construct and test a two input AND.
• C. Design, construct and test a three input AND.
• D. Design, construct and test a three input NAND.
• E. Design a four input NAND.
• F.  Be able to explain why each of your circuits should implement the required circuit.

Hint:
• You can use an LED indicator to show the inputs and/or the outputs in your circuits.
• Use the current-limiting resistor if you use an LED.
• If you connect an LED directly to the output of a gate it will lower the output voltage of the gate.