# PHY405F 2009 EXPERIMENT 7 – INTRODUCTION TO DIGITAL CIRCUITS

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```					                                   PHY405F 2009

EXPERIMENT 7 – INTRODUCTION TO DIGITAL CIRCUITS

Due Date: Thursday, Nov 19th @ 5 pm; Late penalty in effect!

INTRODUCTION

Digital electronics is conceptually much simpler than analogue electronics. There are only
two kinds of states called logic levels. They correspond to the two digital numbers 0 and 1 and are
typically represented by low and high voltages. Logic states within a digital circuit
change very rapidly when an input signal to the circuit transitions between 0 and 1 or
between 1 and 0. Logic is said to be edge triggered. Logic circuits are made up of many high speed
switching transistors. You will investigate one circuit built of discrete components and then move
on to discover the properties of integrated circuits of the transistor-transistor logic (TTL)
family which dates from about 1970!

WHAT TO DO

1. A basic logic circuit of a style no longer in general use but easy to construct in
discrete form is shown here. It is called a 3 input DTL NOT-AND or NAND gate.
The inputs are VA, VB, VC and the output is VOUT. Construct the circuit using a
2N2222/2N3903 transistor and 1N914 diodes. Try to match the resistors to the given
values. You may need two in series for each Logic level. 0 and 1 at the inputs should
be ground and VCC.

Since the three input voltages can be either logic 0 or logic 1, there are 4 unique input
combinations corresponding to 000, 001, 011 or 111 or permutations thereof. Measure
the output voltage for each of these states and established the rules for a NAND gate.
Give a theoretical explanation of your results in terms of the switching characteristics of
the transistor and the voltages and currents in the circuit.
Hold two or the inputs VA and VB at logic 1. Plot a graph of the variation of the input
voltage on VC with the output voltage VOUT and establish the range of input voltages
acceptable for 0 and 1. Using the CRO, measure the speed of transition between 0 and 1
and between 1 and 0 at the output for a step change in voltage at the input. Use the signal
generator to produce a pulse. Trigger the CRO with the same pulse. View it on channel 1
of the CRO and the output on channel 2.

2. Integrated TTL logic comes in dual-in-line plastic packages. The packages contain one or more
independent gates from the collection shown as circuit symbols below. Identify each gate and
fill out the truth table which defines its characteristics.

A number of TTL logical gates are available in the cupboard. In particular, there are
Inverter 7404
2-input NAND gate 7400
2-input NOR gate 7402
4-input NAND gate 7420
4-input AND/OR invert gate 7451
2-input XOR 7486
3. The inverter 7404 is the simplest TTL logic element. The output level is just the inverse
of the input. The circuit is shown below. Find out how it works. In particular, try to work
out the time sequence in which the transistors switch states as the input pulse changes
from 0 to 1 and from 1 to 0.

It helps to recognize that the input transistor is not acting as a switching transistor but
as a pair of steering diodes like this:

Just as you did in question 1, measure and plot the switching characteristics of the
inverter and determine its switching time from state to state. It is most important to
keep VCC between 4.75 and 5.25 V!

The TTL gate differs from the more elementary DTL gate. It as a Totem Pole output
capable of both sourcing and sinking current at its output. Show that it can drive an
LED through a 1000 ohm resistor connected to either VCC or ground to indicate
logical 0 or 1.

4. The circuit shown below checks for a unanimous committee vote. What is the logical
state of the input if the switches are all open? All switches have to be closed for the
LED to light. Show that this is true by building and testing the circuit carefully! Design
the layout of your board so that logic states can be measured easily. Use short lengths
of wire!!! Decouple VCC locally at each chip with a 0.01 microfarad capacitor
between the VCC pin and the ground pin. Be systematic in setting up your VCC and
ground buses.

A logic diagram is by no means unique. Can you think of an alternative scheme using different
elements that will accomplish the same result? How would you change the circuit to detect a
simple majority? Can you test it experimentally? How would you change the scheme to an odd
number of yes votes (the parity problem)?
APPENDIX

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