Something I had long forgotten about, until I recently got a Revo K1 and wanted to show the kids how GBA linking worked. Some cheap link cables from fleabay only have 4 pins connected, and do not have a GND-GND (or even shields) reference. As a quick temporary test, you can connect the ground of the two headphone sockets. Typically you will find them connected like this: M2 (SO) - S3 (SI) M3 (SI) - S6 (GND) M4 (SD) - S4 (SD) M5 (SC) - S5 (SC) You can see this connects GND from the slave end to SI on the master, so the way I fixed it was to tap into the pin 3 wire on the master end and connect it to the shield. This is fairly easy to do: * Slide the outer casing back. * Loosen the cable retention crimp * Cut back the outer insulation 10-15mm. * Wrap the pin 3 wire (white, in my case, BYMMV) around the shield. * Add a dob of solder. * Test, re-assemble.