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Bidirectional Shift Register with Parallel Load A register capable of shifting in one direction only is called a unidirectional shift register. A register that can shift in both directions is called a bidirectional shift register. Some shift registers provide the necessary input and output terminals for parallel transfer. The most general shift register has all the capabilities listed below. Others may have some of these capabilities, with at least one shift operation.

1. An input for clock pulses to synchronize all operations.

2. A shift-right operation and a serial input line associated with the shiftright.

3. A shift-left operation and a serial input line associated with the shift-left.

4. A parallel load operation and n input lines associated with the parallel transfer.

5. n parallel output lines.

6. A control state that leaves the information in the register unchanged even though clock pulses are applied continuously.

A 4-bit bidirectional shift register with parallel load is shown in Fig. 2-9. Each stage consists of a D flip-flop and a 4 X 1 multiplexer. The two selection inputs Sl and So select one of the multiplexer data inputs for the D flip-flop. The selection lines control the mode of operation of the register according to the function table shown in Table 2-4. When the mode control S,So = 00, data input 0 of each multiplexer is selected. This condition forms a path from the output of each flip-flop into the input of the same flip-flop. The next clock transition transfers into each flip-flop the binary value it held previously, and no change of state occurs. When S1So = 01, the terminal marked 1 in each multiplexer has a path to the D input of the corresponding flip-flop. This causes a shift-right operation, with the serial input data transferred into flip-flop Ao and the content of each flip-flop A; - 1 transferred into flip-flop A; for i = 1, 2, 3. When S,So = 10 a shift-left operation results, with the other serial input data going into flip-flop A3 and the content of flip-flop A; +, transferred into flip-flop A; for i = 0, 1, 2. When S1So = 11, the binary information from each input Io through 13 is transferred into the corresponding flip-flop, resulting in a parallel load operation. Note that the way the diagram is drawn, the shift-right operation shifts the contents of the register in the down direction while the shift left operation causes the contents of the register to shift in the upward direction.

Shift registers are often used to interface digital systems situated remotely from each other. For example, suppose that it is necessary to transmit an n-bit quantity between two points. If the distance between the source and the destination is too far, it will be expensive to use n lines to transmit the n bits in parallel. It may be more economical to use a single line and transmit the information serially one bit at a time. The transmitter loads the n-bit data in parallel into a shift register and then transmits the data from the serial output line. The receiver accepts the data serially into a shift register through its serial input line. When the entire n bits are accumulated they can be taken from the outputs of the register in parallel. Thus the transmitter performs a parallel-toserial conversion of data and the receiver converts the incoming serial data back to parallel data transfer.