Lab-9, Basic Digital Gates
Physics U371/372, Electronics for Scientists, Fall 2007
Don Heiman, Northeastern University (11/8/07)

This lab is an introduction to binary gates. Gates are among the simplest integrated circuits. Each gate produces a digital output for one or more digital inputs. Typically, several identical gates are contained in one monolithic chip. You will study the gate behavior of the 74XX series TTL gates. These are the basics building blocks from many electronic logic circuits, and you need to understand their operating conditions. In the process, you will learn how to lay out a simple test circuit on the prototype board. You might be surprised how straightforward most of this is. As part of the preparation for this lab, you should carefully go over the data sheets and understand the truth tables. Data sheets are provided by the chip manufacturers’, which can be found by searching the internet or shown on pages 468-471 of the textbook.

You will need:

• prototype board (with 5V power supply) jumper wire kit
• 2 DVM’s
• LED + 500Ω current-limiting resistor
• SPDT switches
• patch cords
• chips listed below

You can test the output level of the gates with LEDs. An LED in series with a current-limiting 500 Ω resistor on the output lights up when the output is +5V.

There are also SPDT (single-pole double-throw) switches you can use to switch each input between +5V and ground. Note that all inputs must be either at +5V or grounded – they must not be floating.

1. Question: When the output of an AND gate is a 0 or a 1, what are the output voltages relative to ground?

2. Determine truth tables of standard gates: Obtain these five chips in the basic TTL series:

AND 7408

OR 7432

NOR 7402 (Note that the NOR gates in the IC chip are reversed.)

NOT 7404

NAND 7400

Measure the truth table for one gate in each of the above IC chips.

To test the output state, use either a DVM or LED/resistor.

3. Construct both an XOR and an XNOR gate using AND(s), OR(s), and NOT(s).

Determine their truth tables. Show the design in the lab report.