A Flip Flop may store a single bit of information and to store various bits of data we need a series of Flip Flops, which are connected in cascade and such a group of Flip Flops, are termed as Shift Registers. A three-element Shift Register is illustrated in Figure and it is constructed by using three D type Flip Flops, which are linked in cascade form. It may store 3 bits of information. n Flip Flops may store n bits of information.
Figure: 3-element Shift Register using D Flip Flops
The bits may be moved in the shift registers in two ways. Initially, it may be moved into the shift register in serial manner one by one at a time instant beginning from the Most Significant Bit (MSB) or Least Significant Bit (LSB) and it is called Serial Shifting. Note that the MSB (left-most binary digit) and LSB (right-most binary digit) for the data 1110 is 1 and 0, respectively. Secondly all the bits of information may be shifted simultaneously at a time instant and it is called Parallel Shifting. As there are two ways to shift in the data, likewise there are two ways to shift out a data. Shift registers may be classified into four for each case :
- Serial-in, serial out (SISO)
- Serial-in, parallel out (SIPO)
- Parallel-in, parallel out (PIPO)
- Parallel-in, serial out (PISO) Let us study them one by one.