Manchester code: Difference between revisions

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Revision as of 15:16, 20 January 2018

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In [[telecommunication]] and [[computer data storage|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 biascomponent]]. As a result, electrical connections using a Manchester code are easily [[galvanic isolation|galvanically isolated]].

==Background==

Manchester code derives its name from its development at the [[University of Manchester]], where the coding was used to store data on the magnetic drum of the [[Manchester Mark 1]] computer.

PhaseManchester encoding (PE)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]].

== Features ==

Manchester coding is widely used (e.g., in [[10BASE-T]] Ethernet; [[consumer IR]] protocols; see also [[RFID]] or [[near-field communication]]). There are more complex codes, such as [[8B/10B encoding]], that use less [[bandwidth (signal processing)|bandwidth]] to achieve the same data rate but may be less tolerant of frequency errors and [[jitter]] in the transmitter and receiver reference clocks.{{cn|date=November 2015}}

Manchester encodingcoding is a special case of [[binary phase-shift keying]] (BPSK), where the data controls the [[phasePhase (waves)|phase]] of a square wave [[carrierCarrier signal|carrier]] whose frequency is the data rate. SuchManchester acode signalensures isfrequent easyline voltage transitions, directly proportional to generatethe 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 [[Inductive coupling|inductively]] or [[Capacitive coupling|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 [[isolation transformer]] which cannot convey a DC component.

According to [[Cisco]], "Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates".<ref>{{citation |url=http://docwiki.cisco.com/wiki/Ethernet_Technologies |title=Ethernet Technologies |publisher=[[Cisco Systems]] |access-date=2017-09-12 |quote=Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates.}}</ref><!--Difficulties are in meeting [[Title 47 CFR Part 15]] and other RF emissions requirements.-->

Manchester coding is widely used (e.g., in [[10BASE-T]] Ethernet; [[consumer IR]] protocols; see also [[RFID]] or [[near-field communication]]). There are more complex codes, such as [[8B/10B encoding]], that use less [[bandwidth (signal processing)|bandwidth]] to achieve the same data rate but may be less tolerant of frequency errors and [[jitter]] in the transmitter and receiver reference clocks.{{cn|date=November 2015}}<!--Kvng RTH-->

== Features ==

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 [[Inductive coupling|inductively]] or [[Capacitive coupling|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 [[isolation transformer]] which cannot convey a DC component.

== Encoding and decoding ==

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Manchester code always has a transition at the middle of each bit period and may (depending on the information to be transmitted) have a transition at the start of the period also. The direction of the mid-bit transition indicates the data. Transitions at the period boundaries do not carry information. They exist only to place the signal in the correct state to allow the mid-bit transition. The existence of guaranteed transitions allows the signal to be self-clocking, and also allows the receiver to align correctly; the receiver can identify if it is misaligned by half a bit period, as there will no longer always be a transition during each bit period. The price of these benefits is a doubling of the bandwidth requirement compared to simpler [[non-return-to-zero|NRZ]] coding schemes (or see also [[NRZI]]).

=== Manchester encoding as phase-shift keying ===

Manchester encoding is a special case of [[binary phase-shift keying]] (BPSK), where the data controls the [[phase (waves)|phase]] of a square wave [[carrier signal|carrier]] whose frequency is the data rate. Such a signal is easy to generate.

=== Conventions for representation of data ===