Morse code was an early Human means of non-verbal communication.
History and specificsEdit
This system, developed by Samuel Morse, involved set series of dashes and dots, short and long tones or delays in knocks. (TOS episode: "Space Seed"; TOS novel: The Fearful Summons)
While trapped on a Romulan ship, Charles Tucker III had used the modulation pulses to tap out a message to the Enterprise (NX-01) warning them of an upcoming attack. (ENT novel: Kobayashi Maru)
Montgomery Scott made use of Morse code in the liberation of James T. Kirk and company from the USS Enterprise -A's brig. (TOS movie: The Final Frontier)
In the year 2377 when the USS da Vinci was trapped in a tessaract, they were able to get a message to Montgomery Scott using morse code. (CoE eBook: The Art of the Comeback)