Thus with synchronous transmission entire blocks of

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Unformatted text preview: in the same location as the main computer. Its primary purpose is to offload communications processing task from the main computer, thereby the main computer can be dedicated for applications and data processing jobs. It virtually acts like an intelligent terminal or like a secretary/receptionist to the main computer. ASYNCHRONOUS AND SYNCHRONOUS TRANSMISSION Data transmission on a communication line is normally carried out in two different modes - asynchronous and synchronous. Asynchronous transmission is often referred to as start-stop transmission because of its nature, that is, the sender can send a character at any convenient time and the receiver will accept it. This is the characteristic of many terminals. When a terminal is connected to a computer and an operator manually presses keys on the terminal, the time spent between successive keystrokes would vary. Thus in asynchronous transmission, data is transmitted character by character at irregular intervals. In order to enable the receiver to recognize a character when it arrives, the transmitter "frames" each character. Preceding the character is a start bit and following the character will be one or two (depending upon the system) stop bits. Thus for the 7-bit ASCII code, for each character the transmitter transmits the seven character bits, one parity bit, one start bit, and one or two stop bits for a total of either 10 or 11 bits. The concept of the character framed by start-stop bits is illustrated in Figure 17.15(a). Note that in asynchronous transmission, the time between transmitting any two characters can be zero or of any length. Thus the computer might send a continuous stream of characters to the terminal, or characters keyed in by a terminal operator can be sent to the computer as and when the operator keys in a new character. Asynchronous transmission is well suited to many keyboard type terminals. The advantage of this method is that it does not require any local storage at the terminal or the computer as tran...
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