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 flipflops. On the low going edge of clock, the binary input B0, B1, B2, B3 will get loaded into the corresponding flipflops. 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.