74HC595 is a displacement register with 8-bit serial input and parallel output: the parallel output is a three-state output. At the rising edge of SCK, a single row of data is input from SDL to the internal 8-bit shift register and output by Q7 ', while the parallel output is stored at the rising edge of LCK to the 8-bit parallel output register.
When the control signal of the serial data input OE is low enabled, the output value of the parallel output is equal to the value stored in the parallel output register.
When OE is high potential, that is, when the output is off, the parallel output will remain in a high impedance state