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 as computers, for instance, are represented by
two different voltage levels or two different current levels. If the more positive of the two voltage or
current levels represents a logic ‘1’ and the less positive of the two levels represents a logic ‘0’, then
the logic system is referred to as a positive logic system. If the more positive of the two voltage or
current levels represents a logic ‘0’ and the less positive of the two levels represents a logic ‘1’, then
the logic system is referred to as a negative logic system. The following examples further illustrate this
concept.
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5
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 as computers, for instance, are represented by
two different voltage levels or two different current levels. If the more positive of the two voltage or
current levels represents a logic ‘1’ and the less positive of the two levels represents a logic ‘0’, then
the logic system is referred to as a positive logic system. If the more positive of the two voltage or
current levels represents a logic ‘0’ and the less positive of the two levels represents a logic ‘1’, then
the logic system is referred to as a negative logic system. The following examples further illustrate this
concept.
Digital Electronics: Principles, Devices and Applications Anil K. Maini
© 2007 John Wiley & Sons, Ltd. ISBN: 978-0-470-03214-5