Trainer port hardware specs?

[RobC]

This is a question on behalf of a friend:-

"I've asked elsewhere but no replies yet, does anyone know the hardware specs for the trainer port input please? like whats the max ppm peak voltage it can withstand, whats the minimum peak voltage that will reliably trigger it, any restrictions on the impedance of the incoming source, will it accept AC coupled ppm (which momentarily goes negative), or generally, is it protected in any way?
Ta v much
Phil"

Can anyone help or provide a pointer to this info?

Thanks,
Bob