Please refer to this link.
I understand an input to PLC is high if the physical contacts are closed and low otherwise. But in the water level controller sample program in wikipedia referred above high level and low level float switches are open (input low) when water is above them and vice versa. That means for the fill valve to be latched open both inputs should be high. But this sample program shows both input should be low for the valve to be latched. Have I got any fundamental problem?
I understand an input to PLC is high if the physical contacts are closed and low otherwise. But in the water level controller sample program in wikipedia referred above high level and low level float switches are open (input low) when water is above them and vice versa. That means for the fill valve to be latched open both inputs should be high. But this sample program shows both input should be low for the valve to be latched. Have I got any fundamental problem?