4.1.2: Asynchronous and Synchronous Operation

– In some terminals with keyboard input, characters are generated and transmitted singly, one after the other. Given that no two persons type at the same speed, and that individual keystrokes may be separated by different time delays, the stream will be irregularly paced with spaces between each character.

4.1.2: Asynchronous and Synchronous Operation

– In other terminals, the characters are collected until a complete line of text is created, or the return key is pressed, causing the line to be sent as a burst of continuous characters.

4.1.2: Asynchronous and Synchronous Operation

– Whether sent one-by-one as they are generated, or sent line-by-line as each line is completed, each character is framed by a start bit & stop bits.

– The start bit provides a reference from which the receiver measures time in order to sample the succeeding bits approximately in their center.

– The stop bit(s) provides(s) a positive indication of the end of the character

4.1.2: Asynchronous and Synchronous Operation

– Because the data are sent as soon as they are ready, this style of operation is known as asynchronous.

4.1.2: Asynchronous and Synchronous Operation

– In some terminals, particularly those capable of sophisticated processing, characters are run together in form of a block of data.

– This style of operation is known as synchronous.

4.1.2: Asynchronous and Synchronous Operation

– The data block is framed by a start sequence and a stop sequence.

– The start sequence is called header and it contains• Synchronizing• Address• Control information

– The stop sequence is called the trailer and it contains.• Error checking• Terminating information