telecommunication Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...

and data storage, Manchester code (also known as phase encoding, or PE) is a

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...

in which the encoding of each data

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

is either low then high, or high then low, for equal time. It is a

self-clocking signal In telecommunications and electronics, a self-clocking signal is one that can be decoded without the need for a separate clock signal or other source of synchronization. This is usually done by including embedded synchronization information withi ...

with no

DC component DC, D.C., D/C, Dc, or dc may refer to: Places * Washington, D.C. (District of Columbia), the capital and the federal territory of the United States * Bogotá, Distrito Capital, the capital city of Colombia * Dubai City, as distinct from th ...

. Consequently, electrical connections using a Manchester code are easily galvanically isolated. Manchester code derives its name from its development at the

University of Manchester , mottoeng = Knowledge, Wisdom, Humanity , established = 2004 – University of Manchester Predecessor institutions: 1956 – UMIST (as university college; university 1994) 1904 – Victoria University of Manchester 1880 – Victoria Univer ...

, where the coding was used for storing data on the magnetic drums of the

Manchester Mark 1 The Manchester Mark 1 was one of the earliest stored-program computers, developed at the Victoria University of Manchester, England from the Manchester Baby (operational in June 1948). Work began in August 1948, and the first version was oper ...

computer. Manchester code was widely used for

magnetic recording Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...

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 The physical-layer specifications of the Ethernet family of computer network standards are published by the Institute of Electrical and Electronics Engineers (IEEE), which defines the electrical or optical properties and the transfer speed ...

standards and is still used in consumer IR protocols,

RFID Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder, a radio receiver and transmitter. When triggered by an electromag ...

and

near-field communication Near-field communication (NFC) is a set of communication protocols that enables communication between two electronic devices over a distance of 4 cm (1 in) or less. NFC offers a low-speed connection through a simple setup that can be u ...

Features

Manchester coding is a special case of binary phase-shift keying (BPSK), where the data controls the

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

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 In serial communication of digital data, clock recovery is the process of extracting timing information from a serial data stream itself, allowing the timing of the data in the stream to be accurately determined without separate clock information. ...

. The

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 {{no footnotes, date=May 2014 Network isolators are installed as part of a wired Ethernet system as galvanic isolators to reduce the potential for electrical injury and limit the extent of damage due to lightning strikes. Through the applied pri ...

—a simple one-to-one

pulse transformer A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional part ...

which cannot convey a DC component. According to

Cisco Cisco Systems, Inc., commonly known as Cisco, is an American-based multinational digital communications technology conglomerate corporation headquartered in San Jose, California. Cisco develops, manufactures, and sells networking hardware, ...

, "Manchester encoding introduces some 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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

to achieve the same data rate but may be less tolerant of frequency errors and jitter in the transmitter and receiver reference clocks.

Encoding and decoding

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.

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., Andy Tanenbaum). It specifies that for a 0 bit the signal levels will be low–high (assuming an amplitude physical encoding of the data) – 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. This is also known as Manchester II or Biphase-L code. The second convention is also followed by numerous authors (e.g.,

William Stallings William Stallings is an American author. He has written computer science textbooks on operating systems, computer networks, computer organization, and cryptography. Early life Stallings earned his B.S. in electrical engineering from Universi ...

) as well as by

IEEE 802.4 Token bus is a network implementing a Token Ring protocol over a ''virtual ring'' on a coaxial cable. A token is passed around the network nodes and only the node possessing the token may transmit. If a node doesn't have anything to send, the ...

(token bus) and lower speed versions of IEEE 802.3 (Ethernet) standards. 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. If a Manchester encoded signal is inverted in communication, it is transformed from one convention to the other. This ambiguity can be overcome by using

differential Manchester encoding Differential Manchester encoding (DM) is a line code in digital frequency modulation in which data and clock signals are combined to form a single two-level self-synchronizing data stream. In various specific applications, this method is also call ...

Decoding

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 NRZ coding schemes.

Encoding

Encoding conventions are as follows: * Each bit is transmitted in a fixed time (the "period"). * A 0 is expressed by a low-to-high transition, a 1 by high-to-low transition (according to G. E. Thomas's convention – in the IEEE 802.3 convention, the reverse is true). * The transitions which signify 0 or 1 occur at the midpoint of a period. * Transitions at the start of a period are overhead and don't signify data.

References