Discussion

Register consist of a set of flip-flops used to store and transfer binary data in a digital sytem. register can be classified according to the types of input and output data movement. With the two basic forms of data transfer, serial and parallel, there are the following categories of registers:
 

  Parallel-in / Parallel-out (PIPO)

  Serial-in / Serial-out (SISO)

  Parallel-in / Serial-out (PISO)

  Serial-in / Paralellout (SIPO)
 

Each of these types is available in IC form. Therefore, a logic designer can usually find exactly what is required for a given application.
 
 
 

 
 
 
 Figure 1: 74LS194 bit shift register with parallel access.
 
The 74LS194 shown in Figure 1 is a 4 bit bidirectional universal shift register. This MSI shift register has all the types of registers combined in an IC.

 
                  SO & SI                - Mode control inputs (shift right, shift left, parallel load and inhibit clock)
               A, B, C, D             - Parallel data inputs
               QA, QB, QC, QD    - Data outputs
                L                         - Shift left serial input
                R                         - Shift right serial input
                CLOCK                 - Clock signal input
                CLEAR                 - Active low input to clear the outputs.

A more detail description of this IC and other shift registers ICs are available in the TTL manufacturer's Data Book.