The GPI pinout and circuitry are identical on the USD and SYNC I/O.The one thing that has changed is that the power pin on the SYNC I/O's GPIO has been changed from 5V to 12V. This power is provided to power the opto isolators in film tach applications. In the USD it had a polyfuse and could supply up to one amp of current. This isn't always enough voltage to drive film tach optos. SYNC I/O supplies 12V but only up to 100mA. This is enough to power the film tach optos, but it won't light up more than a few LEDs. LEDs need to supply their own power and the GPIO relay should be in series with this power source and the light.