3x inputs for buttons (or any contacts) that switch to GND 5x universal inputs 2x optocoupler inputs
The key inputs have Schottky diodes and pull-up resistors. They are therefore suitable for buttons but also as a PTT input. The diode does not affect PTT circuits.
The five universal inputs have diodes that clamp against 3.3V if the input voltage is too high. If you want to control the inputs with voltages higher than 3.3V, the resistors R31-R35 should be increased accordingly, e.g. to 10k or 100k.
The two optocoupler inputs are used for longer lines that may be subject to overvoltages. There is an overvoltage diode directly behind the input resistor. The supply for the optocoupler is generated by an internal isolated DC / DC converter. There is a common ground, which is necessary for grounding reasons. The entrance is protected, not the mass.