Working Principle of Machine Telegraphy

Working Principle of Machine Telegraphy Points : working principle of machine telegraphy Machine telegraphy started with the 5-unit code system and worked very well. However, the matching of the sending and receiving mechanism was very critical. ‘To overcome this difficulty the start-stop principle was introduced in machine telegraphy.

In the start-stop principle, for each character the transmitter automatically prefixes the group of code elements with a start signal and terminates with a stop signal. Before any character is sent, a start (space) signal of 20 ms duration is transmitted which shows the start of a character. Then the five code signals, each of 20 ms duration (100 ms) are transmitted which is followed by a stop (mark) signal of 30 ms duration, showing the end of a character.

The synchronization of the transmitter and receiver need only to be for the duration of one character. The length of the stop signal is 1.5 times (20 ms x 1.5 30 ms) that of the other units, to allow the receiving mechanism to identify it in the case of continuous transmission. With this system, the minimum character duration is 150 ms. Most machines work at 50 baud, although newer machines can work at 75 baud, and electronic machines can work at 100 baud. The start-stop format for the five-unit code is shown in the figure.