[FAQ] What is ISO1228's current limit feature? What is IEC 61131-2 type 1, 2 and 3 inputs?

IEC 61131-2 standard defines three input types with ON and OFF states based on IV graph below.

For example, type 1 inputs define the ON state as any voltage above 15V and current over 2mA. The device limits the current as voltage increases after the 15V input threshold high (VIHT). Thus, limiting the power loss. If there is no current limit feature, the current increases linearly with voltage and power loss increases exponentially (P=V*I).