Controller Theory
All controllers are very similar in their basic operation. Without being too technical, the machine will sense the difference in voltage when an input device changes its status. For example, by pressing a button you close a switch, alerting the system to a status change, and it responds accordingly.
There are two basic kinds of input devices - Digital and Analogue. A digital device has only two states: on or off. An analogue device reports it's state with a range of values. For example, a digital stick would report only that it has been pushed, where an analogue stick would also report how far and how fast it had been pushed.
A Digital Device:
	Almost all controllers consist of a device for every input. A console pad or arcade panel with digital inputs would include one 'common' wire, and one 'return' wire for each input. You can see from this diagram that until the switch is closed, the signal cannot get through. For all intents and purposes this is how the system works. When you examine any controller, you will see that one line runs to all of the switches, and one line returns to the system from each switch.

An Analogue Device:
An analogue device determines the signal's status in a slightly different manner. Most analogue devices are always returning a signal, but the strength of the signal determines it's status. If you look at the diagram, you can see how the signal is stronger when the device is moved all the way to one end, and weaker when not moved so far. Since all the systems connected to these controllers are digital, any analogue controller will need to have an Encoder - something to encode the analogue signal into a digital one. The mouse on your computer is an analogue device, which encodes its movement and reports to the computer with a digital signal. A basic encoder simply employs an Analogue to Digital Convertor (ADC) which is usually a chip which changes the signal (4.2v) to a digital value. Other encoders may consist of a notched wheel, where lights would be sensed or blocked by the notches indicating movement. This would create a digital signal (the light is either on or off) through the manipulation of an analogue device. This kind of wheel is still called an encoder, even though it is very different from the ADC.

Advanced Topics:
	What actually happens is that the system holds the return line 'high' or at a positive voltage. The resistor (The squiggly bit) allows just a little signal through, and when the switch is closed, prevents the current from flowing unchecked and causing damage. This allows for a more rigidly defined state, either on or off. Using this method the system knows for sure that a controller is attached and that it's state is very definately a positive or zero voltage. If it were simply a switch, the system would sense it only when it was a closed circuit. When the switch was open, the system wouldn't know if this was open or disconnected.

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