ECE 1315 – Digital Systems
Lab Exercise 0 – Introduction to Logic Gates
and LogiScan
As performed by:
Scott Norr
September 9, 2009
Introduction
The 7400 Logic Family uses TTL logic to implement Boolean Logic gates and other digital
devices. The LogiScan Circuit Exerciser is a platform that dynamically tests all states of
combinational logic circuits and displays the results on a personal computer.
Hypothesis: Using a breadboard, some wire, the 7400 series logic family and a Logiscan Circuit
Exerciser, it is possible to implement four Boolean logic functions, minimize the number of
gates used, and verify the accuracy of their outputs.
Equipment
1 – 74LS04 “Hex Inverter”
1 – 74LS32 “Quad OR”
1 – 74LS08 “Quad AND”
1 – Prototyping Board
1 – LogiScan Circuit Exerciser
1 – Personal Computer with LogiScan Software
Procedure
Four logic algorithms, each using two binary variables, were presented for experimentation. Each
algorithm was implemented using the 7400 Logic Family and tested using the LogiScan circuit exerciser.
It was necessary to convert the algorithms to Boolean expressions in order to implement them. The
four algorithms and the corresponding Boolean expressions are listed in Table 1 below:
Table 1: The Four Algorithms and Corresponding Boolean Expressions
Function
Logic Algorithm
Boolean Expression
F0(X1,X0)
F1(X1,X0)
F2(X1,X0)
F3(X1,X0)
F0=1 if X1=1 OR X0=1, else 0
F1=1 if X1=1 AND X0=1, else 0
F2=1 if X1=1 AND X0=0, else 0
F3=1 if X1=X0, else 0
F0=X1 OR X0
F1=X1 AND X0
F2=X1 AND X0’
F3=F0’ OR F1
All four Boolean expressions were implemented simultaneously on a breadboard, using three logic
chips, the 74LS04, ’08 and ’32. Care was taken to minimize the number of logic gates used on these
S.Norr
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chips. For example, an OR gate was used to create Function 0 and was reused as part of Function 3,
using fewer gates than would otherwise be needed. A schematic diagram of the four functions, as
implemented in hardware is shown as Figure 1.
Figure 1: All Four Functions as Implemented in 7400 Series Hardware
A truth table was constructed by hand to verify the proper output of each algorithm. The truth table
was checked against the output of the LogiScan software. This truth table is shown as Table 2 below:
Table 2: A Truth Table for Each Algorithm
X1
0
0
1
1
S.Norr
X0
0
1
0
1
F0
0
1
1
1
F1
0
0
0
1
F2
0
0
1
0
F3
1
0
0
1
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Any errors in the hardware implementation, identified by discrepancies between the truth table and the
LogiScan output were traced and corrected. The above truth table is consistent with the experimental
results generated by the LogiScan output.
Conclusion
This lab exercise provided several learning opportunities. It introduced the experimenters to the 7400
Logic Family, provided basic instruction in wiring logic gates on a prototyping board and required the
proper use of the LogiScan circuit exerciser. In addition this lab initiated the process of learning how to
implement logic expressions in hardware and generating truth tables.
All four logic functions were successfully implemented using the 7400 Logic Family and the outputs
verified with LogiScan. The number of gates used to implement the four logic functions was 6 logic
gates.
Attachments
1.
S.Norr
Lab Exercise Sheet with Instructor Signature
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