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.
That's what I thought at first, too. The DIN connector on the right being the power supply connector, and the DSUB9 being the joystick port (one of them).
BUT: In every C64 model I've seen, there is the power switch right between the DIN and DSUB9. Which, in the photo, is not. So it is not a C64.
It could still be a joystick port, as DSUB9 was the most common joystick port form back then, before the DSUB15 commonly used on PCs for analog joysticks were a thing. Actually, serial ports back then used the DSUB25 connector. DSUB9 "reduced serial ports" came later.
Another common use of DSUB9 was for VGA before the three-row DSUB15. The DIN connector was commonly used for audio but also (composite) video.
My personal guess is that the bottom picture shows the video output of a computer, with the DSUB9 being an old-style VGA and the DIN being audio+composite video.