Page 59 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

A parallel in serial Out (PISO) shift register is used to convert parallel data to serail data.
Parallel-In Parallel-Out Shift Register (PIPO)
The shift register, which allows parallel input (data is given separately to each flip flop and in a simultaneous
manner) and also produces a parallel output is known as Parallel-In parallel-Out shift register. The logic circuit
given below shows a parallel-in-parallel-out shift register. The circuit consists of four D flip-flops which are con-
nected. The clear (CLR) signal and clock signals are connected to all 4 flip-flops. In this type of register, there are
no interconnections between the individual flip-flops since no serial shifting of the data is required. Data is given
as input separately for each flip flop and in the same way, output is also collected individually from each flip flop.

A Parallel in Parallel out (PIPO) shift register is used as a temporary storage device and like SISO Shift register
it acts as a delay element.
Shift Register Counter
Shift Register Counters are the shift registers in which the outputs are connected back to the inputs in order to
produce particular sequences. There are basically two types:
• Ring Counter
• Johnson Counter

Ring Counter
A ring counter is basically a shift register counter in which the output of the first flip-flop is connected to the next
flip-flop and so on and the output of the last flip-flop is again fed back to the input of the first flip-flop, thus the name
ring counter. The data pattern within the shift register will circulate as long as clock pulses are applied. The logic
circuit given below shows a Ring Counter.

Ring Counter Truth Table

Clock pulse Q1 Q2 Q3 Q4

0 1 0 0 1

1 1 1 0 0

2 0 1 1 0

3 0 0 1 1

The circuit consists of four D flip-flops which are connected. Since the circuit consists of four flip-flops the data pat-
tern will repeat after every four clock pulses as shown in the truth table. A Ring counter is generally used because
it is self-decoding. No extra decoding circuit is needed to determine what. State the counter is in.
Johnson Counter

A Johnson counter is basically a shift register counter in which the output of the first flip flop is connected to the
next flip flop and so on and the inverted output of the last flip flop is again fed back to the input of the first flip flop.

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CITS : E & H - Electronics Mechanic - Lesson 9 - 29