This function block checks an analogue input (AI) for exceeding or falls below thresholds. Von greater Voff: The output (Q) is turned ON as soon as the value of the input (AI) exceeds the threshold (Von) and is turned OFF as soon as (AI) falls below the threshold (Voff). Von less Voff: The output (Q) is deactivated as soon as the value of input (AI) exceeds the value of the threshold Voff. It gets activated when the input AI exceeds the value of threshold Von, but is lower than Voff.