Serial communication allows a series of bytes (or ASCII characters) to be sent along a cable. The bytes of data are sent as a series of successive "baud"s. Each baud is a signalling bit one the cable. (not all bauds carry data, some are sued for overhead).
At the sender, a shift register (in either hardware or software) is used to serialise each information byte into the sequence of bits that are then sent using an I/O port and a bus driver to connect to the cable.
At the receiver, the remote system reassembles the sequence of bits to form a byte. This is then forwarded for processing by the link layer, where successive bytes form a frame that may be interpreted.
A clock (timing signal) is also needed at the receiver to clock data into the receive shift register. This must identify the timing of each bit (in practice it is preferable to identify the centre of the bit - since this usually indicates the point of maximum signal power). There are two basic ways that timing may be provided on a communications link:
Non-return to zero (NRZ) encoding is commonly used in slow speed communications interfaces for both synchronous and asynchronous transmission. Using NRZ, a logic 1 bit is sent as a high value and a logic 0 bit is sent as a low value (the line driver chip used to connect the cable may subsequently invert these signals). This works fine for asynchronous communications (where the clock resynchronises every byte), but requires additional methods when used with synchronous transmission.
At the sender a clock is used to drive the shift register, clocking out each bit to the physical layer interface. A clock (timing signal) is also needed at the receiver to clock data into the receive shift register. This must identify the timing of each bit (in practice it is preferable to identify the centre of the bit - since this usually indicates the point of maximum signal power).
There are two modes of transmission: synchronous and asynchronous:
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Prof. Gorry Fairhurst, School of Engineering, University of Aberdeen, Scotland. (2014)