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Friday, November 27, 2009

Serial Data Transfer

In a serial link, the transmitter, or driver, sends bits one at a time, in sequence. One signal required by all serial links is a clock, or timing reference, to control the flow of data. The transmitter and receiver use a clock to decide when to send and read each bit. There are two types of serial-data formats :

- Synchronous Format : Data transfer method in which a continuous stream of data signals is accompanied by timing signals(generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets) spaced by fixed time intervals. After the synchronized characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin. The following is a list of characteristics specific to synchronous communication:
* There are no gaps between characters being transmitted.
* Timing is supplied by modems or other devices at each end of the connection.
* Special syn characters precede the data being transmitted.
* The syn characters are used between blocks of data for timing purposes.

- Asynchronous Format : The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character. Asynchronous, or character-framed, transmission is used to transmit seven or eight-bit data, usually in ASCII character format. Each character has a specific start and end sequence, usually one start bit and one or two end (stop)bits.
When gaps appear between character transmissions, the asynchronous line is said to be in a mark state. A mark is a binary 1 (or negative voltage) that is sent during periods of inactivity on the line. When the mark state is interrupted by a positive voltage (a binary 0), the receiving system knows that data characters are going to follow. The following is a list of characteristics specific to asynchronous communication:
* Each character is preceded by a start bit and followed by one or more stop bits.
* Gaps or spaces between characters may exist.

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