Hi, it seems to me that the optocoupled input section of the KC868-A8 is totally useless, since the same GND is used on both side of the optocoupler (so, you could use a simple mosfet or transistor instead of providing a fake galvanic optoisolation).
Why did you choose to put these optos?
In order to provide a better robustness respect to external disturbancs coming from the external world, you should put an isolated DC/DC converter to have a true different GND on the "user side" of the opto.
Otherwise (like in the below attachment) you should disconnect the board GND and the GND pin on the S{1,2...} input connector, and then let the user to provide an external 12V on the inputs, referred to the "user GND" pin.
If the user does not need for galvanic isolation, then he could simply short the external GND with the board GND and "fallback" to your current solution.
Moreover, you could provide a cheap jumper (that shorts the two GNDs and let the user to switch between ISOLATED MODE and (current) CLEAN CONTACT mode)
Bad design.
Why did you choose to put these optos?
In order to provide a better robustness respect to external disturbancs coming from the external world, you should put an isolated DC/DC converter to have a true different GND on the "user side" of the opto.
Otherwise (like in the below attachment) you should disconnect the board GND and the GND pin on the S{1,2...} input connector, and then let the user to provide an external 12V on the inputs, referred to the "user GND" pin.
If the user does not need for galvanic isolation, then he could simply short the external GND with the board GND and "fallback" to your current solution.
Moreover, you could provide a cheap jumper (that shorts the two GNDs and let the user to switch between ISOLATED MODE and (current) CLEAN CONTACT mode)
Bad design.