World Library  
Flag as Inappropriate
Email this Article

Manchester encoding

Article Id: WHEBN0000287789
Reproduction Date:

Title: Manchester encoding  
Author: World Heritage Encyclopedia
Language: English
Subject: Bit slip, Differential Manchester encoding, Line code, Node-to-node data transfer, School of Computer Science, University of Manchester, RONJA, IBM 1360, Digital Serial Interface, Clock recovery, Token ring
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Manchester encoding

In telecommunication and data storage, Manchester coding (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking, which means that it may be inductively or capacitively coupled, and that a clock signal can be recovered from the encoded data. As a result, electrical connections using a Manchester code are easily galvanically isolated using a network isolator—a simple one-to-one isolation transformer.

Background

The name comes 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.

Manchester coding is widely used (e.g., in 10BASE-T Ethernet (IEEE 802.3); see also RFID or Near Field Communication). 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.

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, allowing the signal to be conveyed conveniently by media (e.g., Ethernet) which usually do not convey a DC component.

Description

Extracting the original data from the received encoded bit (from Manchester as per 802.3):

original data XOR clock   =   Manchester value
     0              0                 0
     0              1                 1
     1              0                 1
     1              1                 0

Summary:

  • 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' convention—in the IEEE 802.3 convention, the reverse is true).[1]
  • 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.

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 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 of a square wave carrier whose frequency is the data rate. Such a signal is easy to generate.

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).[2] 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.

The second convention is also followed by numerous authors (e.g., William Stallings [3]) as well as by IEEE 802.4 (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.

References

See also

 This article incorporates MIL-STD-188).

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.