>>50726391It makes sense when you look at the pinout of the original gameboy link cable. Pin 2 on the port was the serial out, and pin 3 was the serial in. Then the cable was wired so that pin 2 on one gameboy would connect to pin 3 on the other, and vice versa. Perfect for 2 player communication.
On the GBA, that would've been fine for 2 player, but would pose a problem for 3-4 players, as the data would never get sent around the path towards the other players if pins 2/3 were just crossed between player 1 and 2. So instead, they wired the purple connector's output to the gray connector's input (so P1 sends to P2), but gray's output is wired to the middle connector, not the purple input. Whenever someone plugs in another cable, the output from the gray connector on the first cable, is now wired to the input on the gray connector of the 2nd cable (so P2 sends to P3). And then again with another cable, P3 sends to P4.
What happens when you plug two cables into each other's middle connectors, you end up creating a replica of the original GB cable. The gray output on one side is now wired to the gray input on the other side, and vice versa. You could also accomplish this with a single cable by opening up the middle connector, and rewiring it so that the gray's output is wired to the purple's input, bypassing the middle connector. That's pretty much what third party cables did, by having a switch that would mechanically change the wiring inside the middle connector to either use it (for GBA mode) or bypass it (for GBC mode).
