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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?

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From the image below taken from the Siemens S7-200 Manual you can see that there is some protection, they do not fully define the items. But it looks like the opto is there, and resistors for surge suppression and a diode for reverse polarity.

enter image description here

Digikey also has a nice article here.

How much protection you need is really up to you and your particular situation. Including but not limited to Cost / Board Space / Time / etc. For example maybe you won't need to have reverse polarity protection because of the way that the system is installed / used.

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