GSM Instrument TTL triggering

Dear all, i have a question regarding TTL triggering.
I have two instruments, where one is a TTL output (pin 3, on a 15 pins connector) of a GSM call tester and it is used to trigger another instrument at the GATE INPUT (BNC connector) of a spectrum analyser. It seems that i can use a thin wire, connect pin 3 of the call tester directly to the BNC centre conductor of the spectrum analyser without grounding on the wire between the 2 instruments and works perfectly fine without the grounding, can anyone explain to me why it still works even grounding does not exist in such a connection? I look forward to hearing from yo, thanks......

Because probably you get the grounding through the AC main supply, where both instruments are connected.

Hi,

You are playing Russian roulet :))
What if you miss the ground connection to AC mains of one the instruments?
You will burn something for sure.

S.