Chapter 4 - Digital System Fundamentals

Introduction

Plant personnel who are applying digital techniques to process control
applications must understand the fundamentals of digital systems. The
design and implementation of computer-based control systems also
requires a working knowledge of digital principles. In this chapter, we
will discuss the basic concepts of digital signals, numbering systems,
binary logic functions, and relay ladder logic design.

Since the data in process control computers is in binary form (zero or one),
we will turn our attention to binary signals and codes first.

Binary Signals and Codes

If digital techniques are to be used in process control, the measurement
and control signals must be encoded into binary form. Binary signals are
simply two-state signals (on/off, start/stop, high voltage/low voltage,
etc.). The simplest approach to encoding analog data as binary-based digital
words is provided by the American Standard Code for Information
Interchange (ASCII), which uses a pattern of seven bits (ones and zeros) to
represent letters and numbers. Sometimes an extra bit (parity bit) is used
to check that the correct pattern has been transmitted. For example, the
number one is represented by 011 0001, and the number two is represented
by 011 0010. On the other hand, the capital letter A is represented
by 100 0001 and the capital letter B by 100 0010.

Many other methods are used to encode digital numbers in programmable
controllers and digital computers. The most common method is the simple
binary code that we will discuss later in this chapter. First, however, we
will look briefly at numbering systems.