[Alan Anderson]

Again I ask, "what third port?" What are you looking at that shows four of them? Again I tell you that there are only two serial ports available.

Now I ask why a serial port has anything to do with using a switch to control a motor.

A motor usually connects to a Victor speed controller, which sets its output based on signals from one of sixteen pwm outputs on the Robot Controller. Some motors can be connected instead to a Spike relay module, which sets its output based on signals from one of eight relay outputs on the Robot Controller. Switches are typically connected directly to one of eighteen digital inputs on the Robot Controller.