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What are Shift Registers?

by Sonali

The registers are formed using a group of flip flops which are capable of storing data. The shift registers are mainly used for transferring data or storing multiple bits. The data transfer of information can be done either left to right or right to left. The application of clock pulses to this device helps to move data in/out of these registers. N-bits of data can be stored using N number of flip flops. The registers can shift the bits either towards left or right. Hence, can be classified as Shift Left Register or Shift Right registers.

Classification of Shift Registers

Serial-In Serial-Out Shift Register

  • It allows serial input i.e. one bit after another.
  • It produces a serial output known as Serial-In Serial-Out shift register.
  • The data leaves serially bit by bit through the register and has only one output.
  • Therefore called as Serial In Serial Out Registers.
  • The SISO is formed by using 4 D flip flops which are connected in series.
  • The D flip flops are all synchronous.
  • They are also known as SISO.       

Serial-In Parallel-Out shift Register

  • As the name suggests the data input is in serial form but output produced is in parallel form.
  • The input is given one bit at a time.
  • The SIPO is formed from 4 D flip flops as seen below.
  • To RESET these flip flops a clear signal is connected to all of them.
  •  All these flip flops work synchronously. The output from the first flip flop is fed as input to the next.
  • Since they convert serial input data to parallel at output they are mainly used in communication lines.

Parallel-In Serial-Out Shift Register

  • The input through these registers is in parallel form and produces serial output.
  • They are also formed from 4 D flip flops which operate synchronously.
  • The clock pulse is connected to  all the flip flops.
  • All flip flops have input data lines connected to each through multiplexers.
  • The parallel input data along with the output of the previous flip flop are fed as input to the multiplexer.
  • The input of the next flip flop is the output from the multiplexers.
  • It used to convert parallel data to serial data.

Parallel-In Parallel-Out Shift Register

  • The input to these registers is in parallel form and the output obtained is parallel data.
  • There are 4 D flip flops connected as shown below and are all synchronous.
  • All flip flops are connected to clocks and clear signals.
  • The input and output are provided separately to each flip flop. 
  • As no serial shifting operation is required there is no interconnection between any of the flip flops.
  • It acts as a delay element and is used as a storage device.

Bidirectional Shift Register 

  • If a binary number is shifted towards left by one position. Then it is equivalent to multiplying the number by 2. If a binary number towards the right by one position then it is equal to dividing the number by 2.
  • To shift the data in any direction we use a register called a Bidirectional register.
  • These registers help in shifting the data in either of the directions in order to perform the above operation.
  • The data is shifted towards right when the mode is set high i.e. 1
  • The data is shifted towards left direction when mode is reset i.e. 0.
  • The circuit comprises 4 D flip flops operating synchronously.
  • The input data is then connected at two ends of the circuit and depending on the mode and gate selected.


  • They are used for temporary data storage.
  • They are also used for data transfer and data manipulation.
  • The SISO and PIPO registers are used to produce time delay in digital circuits.
  • The SIPO register is also used for conversion of serial data to parallel data.
  • Hence they are used in communication lines .
  • A PISO register is used to convert parallel data to serial data.


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