A paddle is about the simplest controller there is: a knob on a potentiometer, and usually a fire button. Turn the knob and the bat moves; let go and it stays exactly where you left it. That is the whole point. A joystick tells the program which way you are pushing, but a paddle tells it where you want to be, so a player can put a bat in the path of a ball in one movement.
Knobs came before the games industry did. Ralph Baer’s television game prototypes of the late 1960s used hand controllers with knobs, and the Magnavox Odyssey he designed went on sale in 1972 with “vertical and horizontal control knobs” for its ping-pong game, plus a third knob for putting “English” on the ball. Atari’s Pong, the same year, kept the idea and dropped the extra knob.
Fast facts
- Mechanism: a potentiometer (variable resistor) turned by the knob, with end stops.
- Output: absolute position, not direction or speed.
- Early uses: the Magnavox Odyssey and Pong, both 1972.
- Atari’s model: CX30, sold as a pair on one plug; a pair came in the box with early Atari VCS consoles.
- Home computers: read by the TIA on the Atari VCS, POKEY on the Atari 400/800, the VIC chip on the VIC-20 and the SID on the Commodore 64.
- Relative: the spinner turns without stops and reports movement rather than position.
Pong wore them out
Al Alcorn built Pong’s control from “just a simple potentiometer”, and it was the first part to fail. The Atari Book says the prototype’s first breakdown was “because of cheap potentiometers”, not the overflowing coin box of the famous story. Alcorn later did the sums: every quarter meant turning the knob “20 or 30 times”, so “this thing was getting tens of thousands of turns per week”. He ended up buying “a very expensive premier potentiometer from a military contractor so it would last”.
The control shaped the game in a second way. Pong’s bats were positioned through a 555 timer chip, and a limitation in the chip meant they could not reach the top of the screen, “leaving a small gap that a ball can fit through”. Alcorn kept it: “Instead of coming up with a fix, Al left it in as a stalemate breaker.”
How a computer reads one
A potentiometer gives a resistance, and a computer wants a number. The usual trick is to time it rather than measure a voltage: the paddle charges a capacitor, and the machine counts how long the capacitor takes to reach a threshold. A higher resistance means a slower charge and a bigger count. The machines differ in who does the counting.
Atari VCS: the program counts
The VCS’s TIA chip has four “dumped” input ports for up to four paddles. Atari’s Stella Programmer’s Guide describes the cycle: the paddle’s pot charges “a capicitor in the console, and when the capacitor is charged the input port goes HIGH”. The program empties the capacitor by writing a 1 to bit 7 of VBLANK, “then measures the time it takes to detect a logical one at the port”. There is no counter in hardware. The program has to watch the port and keep count itself, on a machine that is already busy drawing the picture line by line. The fire buttons are read separately, from the RIOT chip’s SWCHA port.
Atari’s service manual gives the parts: each paddle holds “a 1 Megohm potentiometer” and a push-button, and “there are two game paddles connected to each I/O plug”. It also admits that the paddles are “non-linear”, in contrast to the rotary driving controller.
Commodore 64: the sound chip counts
On the Commodore 64 the paddle inputs go to the SID. Commodore’s description of the chip says so directly: “Two A/D converters are provided for interfacing SID with potentiometers. These can be used for ‘paddles’ in a game environment or as front panel controls in a music synthesizer.”
The SID does the timing. POTX, register $19 ($D419), returns “values ranging from 0 at minimum resistance, to 255 ($FF) at maximum resistance”, and the value “is always valid and is updated every 512 φ2 clock cycles”, about two thousand readings a second. POTY at $1A does the same for the second paddle. The conversion “is based on the time constant of a capacitor tied from the POT pin to ground, charged by a potentiometer tied from the POT pin to +5 volts”. Commodore recommends 470 kΩ and 1000 pF, and notes that “the larger the capacitor, the smaller the POT value jitter”.
There is a catch. The C64 has two control ports and a pair of paddles on each, but the SID has only two inputs. The CIA chooses which pair the SID sees, through bits 6 and 7 of port A at $DC00, the port the keyboard scan also uses. The Programmer’s Reference Guide warns that “PADDLES ARE NOT RELIABLE WHEN READ FROM BASIC ALONE!!!!” and prints a machine-code routine instead. It switches off interrupts, selects a pair, waits in a short loop so that the SID can make a fresh conversion, reads both registers, and then repeats for the other pair. The paddle fire buttons appear on bits 2 and 3 of the CIA ports.
Other machines
The VIC-20 puts its two converters in the VIC video chip, at $9008 and $9009, each giving “a value from 0 to 255”; the fire buttons come from its two 6522 VIA chips. On the Atari 400 and 800 the POKEY chip reads the paddles, and Atari BASIC’s PADDLE function returns “between 1 and 228, increasing as the knob is turned left”.
The Atari paddles
Atari sold its paddles as the CX30, a “paddle controller pair” on one cable. They were not an optional extra at first. Atari’s owner’s manuals of 1977 to 1980 list “Paddle Controller (Set of 2)” among the contents of the VCS box, next to the joysticks and the Combat cartridge, and tell owners that paddles “should always be plugged into the left controller terminal when only 1 or 2 players are playing”. With a second pair in the right port, up to four people could play.
Later boxes dropped them. A 1982 edition of the owner’s manual says paddle controllers “may be purchased separately”, and by early 1984 Video Games was reviewing an Atari computer version of Activision’s Kaboom! that worked with a joystick, which the reviewer called a “generous gesture” to owners who might not want paddles “that may never be used for any other game”. The reviewer still advised buying them, “since joystick control is nowhere near as precise”.
Paddle games
| Game | Year | Machine | What the paddle did |
|---|---|---|---|
| Pong | 1972 | Arcade | Placed the bat |
| Breakout | 1976 | Arcade | Placed the bat under the wall |
| Kaboom! | 1981 | Atari VCS | Moved buckets to catch falling bombs |
| Warlords | 1981 | Atari VCS | Four players, each guarding a castle |
| Arkanoid | 1986 | Arcade | Moved the Vaus bat |
The Electronic Games 1983 software encyclopedia rated Kaboom! 9 out of 10 overall and listed Warlords as a ball-and-paddle game for one to four players. The three-bat layout of Kaboom! goes back to Atari’s arcade Super Pong, which gave each player three bats, a format The Atari Book says “was later leveraged vertically in games like Atari’s Avalanche and Activision’s Kaboom!”.
Arkanoid in the arcade used a free-turning knob, a spinner rather than a potentiometer. When Taito brought it to the NES, the game came packed with its own controller.
Sources
- Ralph H. Baer, Videogames: In the Beginning (Rolenta Press, 2005): the Odyssey’s control knobs and “English” knob (p. 6); hand controllers with horizontal, vertical and English knobs on TV Game #4 and the Brown Box prototypes (pp. 47, 54).
- The Atari Book (Retro Gamer, 2012): Pong development, the 555 timer, the first breakdown and “cheap potentiometers” (p. 127); Al Alcorn interview on the potentiometer and the military-grade replacement (p. 95); Super Pong and Kaboom! (p. 131).
- Atari, Stella Programmer’s Guide (Steve Wright, 1979), sections 12.1 (dumped input ports) and 5.4 (paddle controllers).
- Atari, 2600/2600A VCS Domestic Field Service Manual (1983), paddle controller and driving controller sections.
- Atari, Video Computer System Owner’s Manual (1977, 1978 and 1980 editions): box contents and plugging in the paddles; Atari 2600 Owner’s Manual (1982): paddles “purchased separately”.
- Micro, October 1979: dealer advertisement listing the “CX30-04 Paddle Controller Pair”.
- Commodore, Commodore 64 Programmer’s Reference Guide (1982): paddles and the four-paddle routine (pp. 346–347); SID description (p. 460);
POTXandPOTY(p. 469); pin description for the POT inputs (p. 472). - Commodore, VIC-20 Programmer’s Reference Guide (1982), “Using Paddles on the VIC”, pp. 248–249.
- Atari, Atari BASIC Reference Guide (1983): the
PADDLEfunction. - Video Games, February 1984, p. 60: review of Kaboom! for Atari computers.
- Electronic Games 1983 Software Encyclopedia: Kaboom! (p. 26) and Warlords (p. 34).
- Retro Gamer issue 43 (2007), p. 67: the NES Arkanoid packed with its own controller, “a smaller version of the ‘spinner’ on the arcade cab”.