How do Atari controllers work?

Controller operation hasn't changed much in 25 years, controllers are basically clever arrangements of switches and potentiometers. In the Atari's case, there is no intervening bus circuitry (like USB or PS/2).

The joystick that comes with the Atari is simply five switches, one each for up, down, left, right, and fire. Power is supplied by the console to the joystick, and when one oo more of the switches are closed, that power is sent back down the wire connected to the switch - agin, one for each direction and fire.

The RIOT or stella then sees power on those particular lines, and knows which controller directions and buttons have been pressed.

So, when you're pushing up and left while firing, the wires for controller 0's up, down, and fire buttons have current flowing through them - when you center the joystick and release the fire button, no current will be flowing on any of the controller lines.

Paddles use a potentiometer (a variable resistor) to alter the level of voltage on the paddle line (there's one line for each of the four possible paddle controllers). It works exactly like a dimmer switch in your house, instead of dimming your light, the changing voltage level is translated into a position along a single axis, which then determines where your character is displayed on the screen.

Look at another section of how an Atari 2600 works:

• Atari 2600 console - processes controller input and cartridge instructions to create video and audio which are output to the TV
• Atari power cable - takes household current (120 Volts, 50-60Hz AC in the US) and transforms it into 9 Volt DC, roughly equivalent to a 9 Volt battery.
• Controllers (joysticks, paddles, keyboard controllers, etc...) - Transform human movement into signals the console can understand.
• RF cable and switch box - Sends the signal from the console to the TV's tuner as if it were a broadcast channel
• Television - The only way to see and hear what the console is producing
• Cartridges - Contain programs that are "run" by the console, which in turn, determine the sights and sounds generated by the Atari.