5- Manchester Signaling
Manchester code is a form
of data communications line code in which each bit of data is
signified by at least one transition. Manchester coding is therefore considered
to be self clocking,
which means that accurate synchronisation of a
data stream is possible. Each bit is transmitted over a predefined time period.
Manchester coding
provides a simple way of encoding arbitrary binary sequences without ever
having long periods without clock transitions, thus preventing the loss of
clock synchronization, or bit errors from low-frequency drift on
poorly-equalized analog links.
Conventions For Representation Of Data
There are two opposing
conventions for the representations of data.
The first of these was
first published by G. E. Thomas in 1949 and is followed by numerous authors
(e.g., Tanenbaum). It specifies that for a 0 bit the signal levels will
be Low-High . with a low level in the first half of the bit period, and a high
level in the second half. For a 1 bit the signal levels will be
High-Low.
The second convention is
also followed by numerous authors (e.g., Stallings) as well as by the IEEE
802.4 standard. It states that a logic 0 is represented by a High-Low signal
sequence and a logic 1 is represented by a Low-High signal sequence.
In summary
- data and clock signals are combined to form a single self synchronizing data stream
- each encoded bit contains a transition at the midpoint of a bit period
- the direction of transition determines whether the bit is a "0" or a "1" and
- the first half is the true bit value and the second half is the complement of the true bit value.
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