Serial In - Serial Out
Shift Registers

A basic four-bit shift register can be constructed using four D flip-flops, as shown below.  The operation of the circuit is as follows.  The register is first cleared, forcing all four outputs to zero.  The input data is then applied sequentially to the D input of the first flip-flop on the left (FF0).  During each clock pulse, one bit is transmitted from left to right.  Assume a data word to be 1001.  The least significant bit of the data has to be shifted through the register from FF0 to FF3.

4-bit SISO shift register
SISO Write

In order to get the data out of the register, they must be shifted out serially.  This can be done destructively or non-destructively.  For destructive readout, the original data is lost and at the end of the read cycle, all flip-flops are reset to zero.


To avoid the loss of data, an arrangement for a non-destructive reading can be done by adding two AND gates, an OR gate and an inverter to the system.  The construction of this circuit is shown below.

Non-destructive read-out

The data is loaded to the register when the control line is HIGH (ie WRITE).  The data can be shifted out of the register when the control line is LOW (ie READ).  This is shown in the animation below.

SISO non destructive output