S0 inputs are active elements. They supply the pulse detector with a signal, but also with an auxiliary power supply. S0 elements can contain a complete electronic package without the complication of an extra power supply connection. The S0 specifications comply with DIN 43864 current and voltage limits. S0 inputs must be able to deliver, at least, a 10mA current when loaded with a resistance of 800. (for supplying the pulse detector). They must also be able to recognize a current of 2.2mA as an "OFF" signal. They are not allowed to deliver more than 27V.

For these reasons, S0 inputs are well suited for use with simple isolated contacts, passive optic coupler elements and transistor elements.

They are often used for control connections (tariffs, resetting, etc.).