Odd problem with limit switch in sidecar

[Doc Wu]

Cut the red wire! No, not the red wire, the blue one. Too late, I cut it.
- reference to every Hollywood movie with a bomb-defusing scene.

Most of us get so used to wiring things up with PWM cables, that we think we need to use all three wires.

Passive limit switches, like the ones in the KOP, and any that are just a set of contacts, only need two wires to operate.

One is your common, which is the black wire. No surprise there.

The other is the white wire, which goes to the signal input on the DIO input. The DIO inputs are forced ("pulled") to 5v internally to the Digital Sidecar, so that with nothing connected, or an open circuit, they read a "high" or logical 1.

To make them "low" or 0, the input needs to be brought to 0v or "common." Shorting the white and black wires through the switch contacts will do this.

There is no need for the red wire, as the input is already connected to it through a current limiting resistor inside the DS.

The only reason to use the red wire on a DIO would be if you had an active sensor that operated off of 5v, such as a photoeye or proximity switch. Most require a higher voltage for operation, though.