An input circuit includes an input line for providing input regarding state of a supply at a first node. An impedance is connected to the input line at a second node for connecting the input line to ground. A Zener diode can be connected in the input line in series between the first and second nodes to impede current flowing through the Zener diode below the Zener voltage thereof in a direction from the first node to the second node, and to allow current above the Zener voltage thereof in the direction from the first node to the second node, wherein the input line is free of any node connecting a power voltage source to the input line between the first and second nodes.