This is a tri-state inverting buffer (states: HIGH,LOW,HIGH-Z)
truth table:
E (diode cathode) Din Output
0 X (don't care) 0 (HI-Z)
1 0 1
1 1 0
The hi-z or high impedance state is when the output is due to E=0 both transistors on totem pole don't conduct thus giving hi-Z state.
