Morse code

Morse code was a antiquated means of visual or audio communication via primitive technology developed on Earth, during the 19th century, such as telegraphs. 

In 2152, Hoshi Sato attempted to use Morse code to communicate with Jonathan Archer while she experienced a hallucination during her first experience aboard a transporter. 

In 2267, a "general call" Morse code was transmitted automatically by the SS Botany Bay and the code was received by the USS Enterprise (NCC-1701). 

In 2287, Montgomery Scott tapped out "stand back" in Morse code on the back of the USS Enterprise (NCC-1701-A)'s brig moments before blasting a hole through the wall freeing Leonard McCoy, Spock and James T. Kirk. 

In 2371, the USS Voyager discovered an Morse code signal for SOS (··· ­­­­­­­­−−− ···, three dots, three dashes, three dots) emanating from a planet in the Delta Quadrant. 

In 2377 during the third segment of the Antarian Trans-stellar Rally Harry Kim had to alert B'Elanna Torres and Tom Paris on the Delta Flyer that their converter was rigged to explode when they crossed the finish line but Irina had damaged the comm system. He was able to alert them using a modulating pulse modified to mimic the Morse code he and Paris had used in the "Captain Proton" holonovels – "converter rigged: eject."