It has inputs for sensors, and outputs for actuators . The internal logic of the RCX translates the electrical signals from, say, a touch sensor, into a form that allows it to respond to a stimulus according to its program. Below are simplified diagrams of how the RCX works as a sensor interface. Each sensor has a spring which pushes a yellow button outwards. Look at the sensor on the left.
Nothing is touching it, so the spring pushes the button outwards.
The conductor on the bottom of the button is touching the two conductors attached to the outer wires. Now look at the sensor on the right. The button is pushed in, so its conductor is touching the other two wires. Depending on how you have programmed your RCX, it will send signals to the actuator to turn on the motor. It's what is called a switch .