Parallel In - Serial Out
Shift Registers

A four-bit parallel in - serial out shift register is shown below.  The circuit uses D flip-flops and NAND gates for entering data (ie writing) to the register.

4-bit PISO register

D0, D1, D2 and D3 are the parallel inputs, where D0 is the most significant bit and D3 is the least significant bit.  To write data in, the mode control line is taken to LOW and the data is clocked in.  The data can be shifted when the mode control line is HIGH as SHIFT is active high.  The register performs right shift operation on the application of a clock pulse, as shown in the animation below.