Hi

it is not a prototocol, but really a signal which tells the internal clock that when the pulse is received it is 0ms, or 0s and 0ms. I dont remember if this is PPM (pulse per minute) or PPS (pulse per second) for this case.

you must adjust the internal clock manually once (via Digsi) and then the pulse will keep the clock synchronized

Pulse per minute. It's described in the system manual.