Digital Electronics: Principles, Devices and Applications

4

Logic Gates and Related Devices

Logic gates are electronic circuits that can be used to implement the most elementary logic expressions, also known as Boolean expressions. The logic gate is the most basic building block of combinational logic. There are three basic logic gates, namely the OR gate, the AND gate and the NOT gate. Other logic gates that are derived from these basic gates are the NAND gate, the NOR gate, the EXCLUSIVEOR gate and the EXCLUSIVE-NOR gate. This chapter deals with logic gates and some related devices such as buffers, drivers, etc., as regards their basic functions. The treatment of the subject matter is mainly with the help of respective truth tables and Boolean expressions. The chapter is adequately illustrated with the help of solved examples. Towards the end, the chapter contains application-relevant information in terms of popular type numbers of logic gates from different logic families and their functional description to help application engineers in choosing the right device for their application. Pin connection diagrams are given on the companion website at http://www.wiley.com/go/maini_digital. Different logic families used to hardware-implement different logic functions in the form of digital integrated circuits are discussed in the following chapter.

4.1 Positive and Negative Logic

The binary variables, as we know, can have either of the two states, i.e. the logic ‘0’ state or the logic ‘1’ state. These logic states in digital systems such ...