Combinational Logic Circuits and Functional Devices

The logic gates which form the basic building blocks of a digital system are designed to perform simple logic operations. A single logic gate is not of much use unless it is connected with other gates to collectively act upon the input data. Different gates are combined to build a circuit that is capable of performing some useful operation like adding three numbers. Such circuits are known as Combinational Logic Circuits or Combinational Circuits. An Adder Combinational Circuit that is able to add two single bit binary numbers and give a single bit Sum and Carry output is shown. Figure 1.7.

Implementing large digital system by connecting together logic gates is very tedious and time consuming; the circuit implemented occupies large space, are power hungry, slow and are difficult to troubleshoot.

Digital circuits to perform specific functions are available as Integrated Circuits for use. Implementing a Digital system in terms of these dedicated functional units makes the system more economical and reliable. Thus an adder circuit does not have to be implemented by connecting various gates, a standard Adder IC is available that can be readily used. Other commonly used combinational functional devices are Comparators, Decoders, Encoders, Multiplexers and Demultiplexers.