this post was submitted on 19 Dec 2023
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Commodore 64 and Atari 2600 here.
That bottom one is the C64 joystick port...
That's a serial port, son.
Wasn't the C64 joystick port serial? Or has 40 years muddled it?
They're both DE-9 connectors, but the C64 joystick port is not serial. Each direction on the stick and the fire button are connected to one of the pins on the connector, and a when direction (or the fire button) is pressed that pin has a voltage applied. So effectively it's a parallel connection.
In a serial connection one of the pins is "send" pin and another is "receive". The device then sends data as a bitstream over those two pins.
No, the C64 (and other computers of the time) used a DSUB9 for the joystick connector. Top row 1-5: Up, Down, Left, Right, PaddleB; bottom row 6-9: Fire, +5V, GND, PaddleA. This was the common layout across all systems back then. The DSUB15 analog joystick port was later. And no, it was no serial port, just raw IO pins.