A multiplexer is Many to one data selector. A multiplexer selects one of the many data available at its input depending on the bits on the select line. For 2n inputs, there are n select lines that determine, which input is to be connected to the output. A multiplexer is used for Parallel to serial data conversion with the help of a counter as shown: [ alt="F1 S.B Deepak 28.12.2019 D21" src="//storage.googleapis.com/tb-img/production/19/12/F1_S.B_Deepak_28.12.2019_D21.png" style="width: 260px; height: 298px;"> The Q-outputs of the modulus-8 counter is connected to the data select inputs of an 8- multiplexer. The first byte (8-bit) of parallel data is applied to the multiplexer inputs. As the counter goes through a binary sequence from 0 to 7, each bit beginning with D0 is sequentially selected and passed through the multiplexer to the output line. After 8 clock pulses, the data byte is converted to a serial format and sent out on the transmission line.