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Concept
Manchester code
Summary
In telecommunication and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no DC component. Consequently, electrical connections using a Manchester code are easily galvanically isolated.
Manchester code derives its name from its development at the University of Manchester, where the coding was used for storing data on the magnetic drums of the Manchester Mark 1 computer.
Manchester code was widely used for magnetic recording on 1600 bpi computer tapes before the introduction of 6250 bpi tapes which used the more efficient group-coded recording. Manchester code was used in early Ethernet physical layer standards and is still used in consumer IR protocols, RFID and near-field communication.
Manchester coding is a special case of binary phase-shift keying (BPSK), where the data controls the phase of a square wave carrier whose frequency is the data rate. Manchester code ensures frequent line voltage transitions, directly proportional to the clock rate; this helps clock recovery.
The DC component of the encoded signal is not dependent on the data and therefore carries no information. Therefore connections may be inductively or capacitively coupled, allowing the signal to be conveyed conveniently by galvanically isolated media (e.g., Ethernet) using a network isolator—a simple one-to-one pulse transformer which cannot convey a DC component.
Manchester coding's data rate is only half that of a non-coded signal, which limits its usefulness to systems where bandwidth is not an issue, such as a local area network (LAN).
Manchester encoding introduces difficult frequency-related problems that make it unsuitable for use at higher data rates.
There are more complex codes, such as 8B/10B encoding, that use less bandwidth to achieve the same data rate but may be less tolerant of frequency errors and jitter in the transmitter and receiver reference clocks.
Official source
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