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-slop (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 as150 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 formal for the five-unit code is shown in the figure.
Merits And Demerits Of Machine Telegraphy
- The machine telegraph uses the equal length code.
- The synchronization of the sending and receiving mechanism is made with the help of start-stop signals.
- The machine telegraphy is speedier and efficient.
- In machine telegraphy a local record of the message can be Recorded on the paper as well as on perforated tape in the coded form.
- With start-stop mechanism the machine can go out of synchronization for one character because each character is independent the previous one.
- In machine telegraphy any disturbance on line upsets me Synchronization.
- The speeds of transmitter and receiver are be kept with Close limits.
- Speed of signal transmission through manual means is limited by Speed keyboard speration by the operator.
- The transmission media also introduces some amount of distortions.