Chapter 3 - Digital Systems

Abstract

Digital systems work with inputs, which are essentially just off/on signals, with the two signal levels represented by 0 and 1. These are termed binary digits. The number system used for everyday calculations is the denary or decimal system. This is based on the use of 10 digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. When working with PLCs, other base number systems are also used; for example, input and output addresses are often specified using the octal system, that is, base 8. However, the PLC itself works with binary numbers. This chapter introduces various number systems. A Combinational logic systems take binary inputs and combine them to give a binary output. The relationship between the inputs and the output can be described by truth tables. With such systems, the output of a particular combination of inputs is determined only by their state at the instant of time concerned. However, with sequential logic systems the output is influenced by the history of the past inputs as well as by the present inputs. Both combinational logic and sequential logic systems are introduced in this chapter.