Page 266 - Engineering Digital Design

CHAPTER 6

Nonarithmetic Combinational

Logic Devices

6.1 INTRODUCTION AND BACKGROUND

It is the purpose of combinational logic design to build larger, more sophisticated logic
circuits by using the most adaptable and versatile building blocks available. The choice of
discrete gates as the building blocks is not always a good one, owing to the complex nature
of the circuits that must be designed and to the fact that there are integrated circuit (1C)
packages available that are much more adaptable. It is the plan of this chapter to develop
these building blocks and demonstrate their use in construction of larger combinational
logic systems. Brief discussions of the various device performance characteristics and a
design procedure are provided in advance of the logic device development.

6.1.1 The Building Blocks
It is well understood that the digital designer must be able to create combinational circuits
that will perform a large variety of tasks. Typical examples of these tasks include the
following:

Data manipulation (logically and arithmetically)
Code conversion
Combinational logic design
Data selection from various sources
Data busing and distribution to various destinations
Error detection

To implement circuits that will perform tasks of the type listed, the logic designer can
draw upon an impressive and growing list of combinational logic devices that are com-
mercially available in the form of 1C packages called chips. Shown in Fig. 6.1 is a partial
listing of the combinational logic chips, those that are of a nonarithmetic type (a) and those
that are arithmetic in character (b). Only the devices in Fig. 6.la will be considered in this
chapter.
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