The 595 function includes latched output registers, which allow the serial shift registers to change while holding the outputs constant. Figure 1-3 illustrates the operation of the 595 function. On the rising edge of the SRCLK input, the data at SER will be loaded and the serial registers will shift by one location as indicated by the arrows in the left diagram of Figure 1-3. To be clear, the value at the SER input will be loaded into the first serial register, the value that was in the first serial register will be loaded into the second serial register, the value that was in the second serial register is moved to the third serial register, and so on. The value that was in the last register, which was previously accessible from the Q_H’ output pin, is “shifted out,” or, in other words, it is overwritten.

On the rising edge of the RCLK input, illustrated in the right diagram of Figure 1-3, the data in the serial registers will be copied to the output registers. The shift registers will still retain the same data in the same locations after this operation is complete.

It is possible to operate this device with SRCLK and RCLK directly shorted together. In this mode of operation, the previous data in the shift registers is first sent to the outputs and then is shifted to the next positions. In this way, the outputs will always be one clock pulse behind the serial register values. To be clear, it will take nine pulses to send eight bits of data to the outputs, and those values will be read into the device with the first eight pulses.

The output Q_H’ comes directly from the last internal shift register, which allows it to be used for daisy-chaining devices together.