Parallel Input Serial Output

Introduction

  • Data bits are entered in parallel fashion.

  • The circuit shown below is a four bit parallel input serial output register.

  • Output of previous Flip Flop is connected to the input of the next one via a combinational circuit.

  • The binary input word B0, B1, B2, B3 is applied though the same combinational circuit.

  • There are two modes in which this circuit can work namely - shift mode or load mode.

Load mode

When the shift/load bar line is low (0), the AND gate 2, 4 and 6 become active they will pass B1, B2, B3 bits to the corresponding flip-flops. On the low going edge of clock, the binary input B0, B1, B2, B3 will get loaded into the corresponding flip-flops. Thus parallel loading takes place.

Shift mode

When the shift/load bar line is low (1), the AND gate 2, 4 and 6 become inactive. Hence the parallel loading of the data becomes impossible. But the AND gate 1,3 and 5 become active. Therefore the shifting of data from left to right bit by bit on application of clock pulses. Thus the parallel in serial out operation takes place.

Block Diagram