RS232 external trigger


We try to use Cubase for research purpose. We would like to trigger the recording from a serial (RS232) or parallel (TTL) / or usb (ascii “s”) signal. Being more an engineer than a musician I am not so familiar with the terminologies used in cubase manual (word clock, midi clock, etc).
As I understood in the Synchronisation chapter of the cubase manual, the purpose of synchronisation is to synchronise for instance video and sound during the whole recording.
Our problem is in a way more simple, we just want to trigger the recording.
Has this problem already been solved before and how?
Otherwise, is it solvable?
Thank you,