This function block realizes a hysteresis, i.e. a threshold switch with positive feedback.

It has one input signal for the higher threshold value and one for the lower threshold value. If the input signal Value reaches or exceeds the limit set by UOn, the output Q will set to HIGH. If Value reaches or falls below UOff, the output Q will set to LOW.


Name I/O Type(s) Function
Value I FLOAT Input signal
UOff I FLOAT Lower (switch-off) limit
UOn I FLOAT Higher (switch-on) limit
Q O BIT Switch output


If output Q is set to HIGH, the function block will be highlighted.


For the normal operation of the hysteresis, the switch-on limit has to be higher than the switch-off limit. Otherwise the function block will constantly switch, if Value is between the given limits.