CMPEN270 – Digital Design Practice
Lab2 - Implementation of Boolean Equations with Discrete Gates
Penn State Erie, The Behrend College
1 Discussion
This lab will examine the design of a circuit from a word statement to a physical implementation using
discrete gates.
2 Pre-Lab
Design a circuit to engage the ignition of a car. The car will NOT start when the key is turned, if
 the doors are closed and the seat belts are unbuckled or
 the seat belts are buckled and the parking brake is on or
 the parking brake is off and the doors are not closed
This ignition circuit has one output, S and three inputs, B, D, and P. These variables are defined as
follows:
 If NS = 1 then the car will not start otherwise the car starts.
 If S = 1 then the car starts otherwise the car does not start.
 B = 1 when the belts are buckled, otherwise the belts are unbuckled.
 D = 1 then the door is closed, otherwise the door is open.
 P =1 when the brake is on, otherwise the brake is off.
Truth table
B
0
0
0
0
1
1
1
1
D
0
0
1
1
0
0
1
1
P
0
1
0
1
0
1
0
1
NS
S
F
G
Table 1: Truth table for the starting circuit and two circuit diagrams below.
The ignition circuit for S will be realized in two different ways.
1) Since the word statement focuses on when the car will NOT start, first complete the truth table for the
NS column (when the car will Not Start). Complete the S column by negating the entries in the NS
column.
2) Use the schematic below and generate the truth table for the output F in the table.
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Figure 1: The schematic for the starting function.
3) Each of the AND gates in the schematic (A1 and A2) is responsible for forcing the output to 1 for a
particular input. For each and gate identify the associated input which it causes the output to equal 1.
4) Use the schematic below and generate the truth table for the output G in the table.
5) Each of the AND gates in the schematic (A1 and A2 and A3) is responsible for forcing the output to 1
for two inputs. For each and gate identify the associated inputs which it causes the output to equal 1.
3 In-lab
Use integrated circuits to wire the circuit for F using the integrated circuits and breadboards provided.
Check the circuit’s operation by changing the inputs to all possible combinations and observing the
output. Confirm that the hardware works according to the truth table. Have the instructor sign your prelab to verify the proper operation of the circuit.
Appendix A: 74xxxx Data Sheets
The circuit will be constructed using logic devices constructed from semiconductor devices. Devices with
similar properties are grouped into logic families (ECL, CMOS, and TTL). This lab will employ the TTL
family because they are relatively inexpensive and durable. ). Each TTL IC has a part number. Each part
number begins with an alphabetic prefix, indicating the manufacturer. The prefix is followed by a two2
digit code that indicates that the IC is either of military grade (54) or commercial grade (74). The next
component (if any) of the part number is an alphabetic code for the TTL series. Next is a number
indicating the type of logic device (for example, AND gate, inverter, etc.). Finally, there is an alphabetic
suffix indicating the package type.
Integrated Circuits
Because of advances in semiconductor technology, it is now possible to place millions of electrical
components on a single piece of silicon, called integrated circuits (IC). The ICs come in a variety of
plastic or ceramic packages with connecting pins designed for mounting on printed circuit boards.
DIP Package
The most useful IC package for building prototype circuits on prototyping boards is the Dual In-line
Package (DIP) which has two rows of pins that conveniently insert into the breadboard. Pins are labeled
numerically starting at 1. Each DIP package is marked to identify pin 1. In most cases, the package has a
notch at the top or a small dimple adjacent to pin 1.
Data Sheets
Among many other things, the manufacture data sheet describes the relationship between the internal
gates and the pins of the chip. For example, the first row of Table A.1 shows the truth table, logic
diagram, and pin out for a 74LS32. Pin 7 and 15 must be attached to ground and power (5v) in order for
the chip to work. The chip has four OR gates. The first has inputs on pins 1,2 and its output is available
on pin 3.
Quad 2-input OR gate
Triple 3-input AND gate
74xx32
74xx11
3
Hex Inverter
74xx04
Table A.1: Excerpts from 3 data sheets for common 7400 series ICs.
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