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This chapter introduces formats and protocols used in asynchronous serial
communications including low-level data formats, encoding methods for
binary and text data, and protocols to ensure reliable data transfer.
A serial port output that functions as a transmitter, or driver, sends bits one at a
time to a serial-port input that functions as a receiver, typically on a different
computer. The cable between the computers typically has a dedicated data path
for each direction. Some serial interfaces have a single, shared data path for both
directions, with the transmitters taking turns.
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The serial communications described in this book use an asynchronous protocol.
In an asynchronous protocol, the interface doesn??™t include a clock line.
Instead, each computer provides its own clock to use as a timing reference. The
computers must agree on a clock frequency, and the actual frequencies at each
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computer must match within a few percent. A transmitted Start bit synchronizes
the transmitter??™s and receiver??™s clocks.
In contrast, in a synchronous protocol, the interface includes a clock line typically
controlled by one of the computers, and all transmitted bits synchronize to
that clock. Each transmitted bit is valid at a defined time after a clock??™s rising or
falling edge, depending on the protocol.
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