# Shift register Tri-State and ULN2803A

I was planning to use the SN74HC595 shift register in association with the ULN2803A and MIC2981 to switch on and off some loads based on the data of the shift register.

What happens if I disable the output of the shift register (using the OE line) and lines Qa to Qh go into high impedance state ?

Would the ULN or MIC chips read the high impedance input as randomly high/low, ie, switching on and off the loads randomly ?

 ----------     Qa Qh    ----------
|          |  --------  |          |  -------- Load A
|  Shift   |  --------  | ULN2803A |  -------- Load B
| Register |  --------  |    or    |  --------
|          |  --------  | MIC2981  |  --------
|          |  --------  |          |  -------- ...
|          |  --------  |          |  --------
|          |  --------  |          |  -------- Load H
----------              ----------
|OE

• You happen to have chosen bipolar input drivers, where high Z input is OK, and behaves as 'off'. There also exist cmos input drivers, where the same does not apply. – Neil_UK Jul 20 '18 at 8:49