Take for example a Siemens S7 24V digital input. I read a couple of articles and also questions on SE and it seems there should be (maybe integrated, maybe as a discrete part):
- optocoupler to electrically isolate
- TVS diodes to shunt/clamp spikes
- surge resistor to limit inrush currents
- capacitor to absorb spikes
Is this correct?
How is an industrial 24V SPS input protected?
What protection and in which order would this input have?
Does somebody have an layout of such an input to study?