PHD2 computes the guide requirements and sends them through ascom to the mount. This is termed pulse guiding. The data is sent to the mount via the communication method ASCOM has to the mount. Please, Please, Please save yourself endless grief and go serial. It is easy if you get the recommended gear.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired