The V5.88 Ultra has 10 optocoupler-isolated inputs (OPT11–OPT20) arranged in two rows along the top edge. All ten channels are electrically identical, and every input header carries the same three pins: Signal / GND / SW-VCC.

• The board sources the current, not your sensor. Current leaves SW-VCC, passes through the on-board resistor and the optocoupler LED, and exits at the Signal pin. An input triggers only when your device pulls Signal down to GND.
• No external resistor is needed — not even at 24 V. Each channel already carries its own 2.2 kΩ series resistor (R130–R139) on the board.
• Wrong polarity will not damage the board. A 30 V Schottky diode (RB521S-30) sits in series with every channel and blocks reverse current, so putting a positive voltage on a Signal pin simply fails to trigger instead of destroying the optocoupler. That covers any voltage this board can itself supply; never feed a Signal pin from a separate supply above 30 V.
• The SW-VCC rail feeds all ten optocouplers, so the jumper must always sit in one of its two positions — even when you only use dry contacts.

Any dry-contact switch or button: no polarity, no supply, just an open/closed contact. Works on every one of the ten inputs.

1. Leave the SW-VCC jumper at its default 5 V position.
2. Wire the switch between the Signal pin and the GND pin. Only two wires are used — leave the SW-VCC pin of that header unconnected.
3. Prefer the normally-closed contact where practical, so a broken wire shows up as a triggered endstop instead of a dead one.
4. Match the YAML polarity to the contact you chose, then confirm with $limits.

motor0:
  limit_neg_pin: gpio.42:low
  limit_pos_pin: gpio.41:low

A powered endstop repeats far more precisely than a bare micro switch. The PiBot Optical Endstop V5 and Mechanical Endstop V5 are built for this input: when triggered, their output pulls Signal down to GND, which is exactly what the optocoupler needs.

1. Set the SW-VCC jumper to 5 V (left position, factory default).
2. Sensor +5V → SW-VCC pin.
3. Sensor Signal → Signal pin.
4. Sensor GND → GND pin.
5. Check the module LED and the matching board input LED, then verify with $limits.

Inductive and capacitive proximity sensors give the best noise immunity on larger machines. They run from the same Vmot-main rail that feeds the motors.

1. Set the SW-VCC jumper to Vmot-Main (right position).
2. Power the controller from the 12 V or 24 V supply the sensor needs. Vmot-main and SW-VCC must never exceed 24 V.
3. Sensor brown / V+ → SW-VCC pin, blue / V− → GND pin, black / output → Signal pin.
4. Verify each sensor with $limits before it is anywhere near a moving axis.

motor0:
  limit_neg_pin: gpio.40:low
  limit_pos_pin: gpio.39:low

• Dry contacts mix with anything. They draw no supply of their own, so standard switches can share the board with either a 5 V or a 12–24 V group.
• 5 V and 12–24 V powered sensors cannot be used together on this board, because they would need two different SW-VCC settings at once.
• If you really need both, power the second group from its own suitable supply, leave those sensors' SW-VCC pins unconnected, and join grounds only where the equipment design permits. When in doubt, use isolated interface modules.