An Atari style paddle was used on Atari 2600 consoles and Commodore 64. There is a noticeable difference between these two models; Atari use 1Mohm variable resistor while Commodore are 460Kohm. This change the transfer curve to be read and thus different values has to be put in the firmware.
Because paddles are analog controllers, they are read in a very different fashion that numeric controllers (like joysticks). Original design of these controllers was made in early ’70 and where meant for rather slow 8-bit computers with limited I/O. Don’t even think about ADC (analog to digital converters). The value of the variable resistor was measure by an RC circuitry controlled by the CPU on a simple TTL I/O port. Trick was to start a timer after forcing the capacitor to ground and discharge it. Then the port was released to high impedance thus charging the capacitor at the R times C rate. Once the TTL one level reached, timer was read. Value of the timer is directly proportional to the R value of the pot, thus the knob position. Because the resistance of the pot was lot greater than the one use in the RC circuit (1M vs 1.8K), charging curve was almost linear, thus giving a linear control to the player.
|3||PADDLE 1 BUTTON|
|4||PADDLE 2 BUTTON|
|5||PADDLE 2 POT|
|9||PADDLE 1 POT|