The parallel-in, serial-out shift register is used in the output process for serial data.
A word is loaded (all at once) into the shift register from the computer.
Then, bits are sent to the modem from the right end of the shift register one at a time.
Parallel-in, parallel-out shift register chips are limited to 4 or 5 bits because of the number of connections required.
The 7495 is very similar to the 74166 in control structure, except that it has a separate clock input for shifting and for loading.
In most computers, there are both left and right shift and rotate* instructions.
To implement these, we might use a right/left shift register (such as the 74194, a synchronous parallel-in, parallel-out 4-bit shift register).
For this, a three-way multiplexer is needed at each bit, since that bit can receive the bit to its left, the bit to its right, or the input bit.
A truth table describing the behavior of the shift register is shown in Table 8.1.
Bits are numbered 1 to 4 from left to right.