All Game Boy Advance link cables that I have seen are lacking the VCC pin. For anyone trying to build sync or control gear for a Game Boy Advance console, this represents a problem in that an external power source must be used. This is usually sourced from an AC/DC supply, a battery or battery pack, or from a computer via USB depending on the functionality of the device.
If only it were possible to use a GBA cable to supply power to an external device, thereby negating the need for an external power supply of any kind...
Trash80 pioneered an old-style Game Boy / Game Boy Color cable modification. I am happy to report that this same modification also works with a Game Boy Advance Cable.
The process is quite simple and quick to do, but is a little annoying considering how small the link port and pins are. Simply use the smallest size of flat head jewelers screwdriver, and pry back the outside plastic shell of the link connection on the cable. There are two small, metal teeth that stick out from the connector and hold the plastic shell in place.
Thankfully, the teeth are easy to push down with the screwdriver and then it is possible to lift the outer lip of the plastic shell up and over the tab. This allows the plastic shell to be pulled further up along the GBA cable, exposing the link connection's metal outside. All of the wires are held in place by two metal tabs that hold the wires together as they exit the port connection. Using the screwdriver, pry the metal tabs upwards, freeing the cables.
Select a cable / pin that you don't need for the application you are using. Because I was making a sync device for Nanoloop 2, I chose to swap the SC pin for the VCC pin. More information about the GBA link cable can be found here. Whilst pushing down on the pin from the front and inside of the connector, pull the wire of the connector from behind the link connection. It should be possible to pull out the pin by the wire.
Now, simply insert the pin (with the correct orientation) back into the link port, this time in the position of the VCC pin (pin 1). Re-crimp the metal tabs, replace the plastic shell and try and lift up the metal teeth holding the plastic shell in place around the connector. Measuring between VCC and ground, there should be 3.3V which can be used to power components such as microcontrollers, LEDs etc.