This system is a comparator with three states : 0, 1 and X.
The output signal depend of the input signal. This system have two limits : low and high.
To output a high signal, the input must be higher than the high limit. To output a low signal, the input must be lower than the low limit.
The value of the limits is given by the resistor bridge:
Ue- = -Vcc.(R1/R1+R2)
Ue+ = Vcc.(R1/R1+R2)
R1 is the resistor between ground and V+.
R2 is the resistor between V+ and the output.
The output signal is inverted : an high input give a low output ; a low input give an high output.
