Manchester code

from Wikipedia, the free encyclopedia

The Manchester code (also known as phase encoding (PE) or directional clock script ) is a line code that receives the clock signal during encoding . The binary phase position (either 0 ° or 180 °) of a square-wave signal is determined by a bit sequence. So that the Manchester code is a form of binary phase shift keying (engl. Binary phase-shift keying ). In Manchester code is u. a. used for 10 Mbit / s Ethernet .

It follows from the phase shift that the edges of the signal that occur in the middle of a period of the clock signal carry the information.

Coding of the bit string 10100111001 in the two possible definitions of the Manchester code

General

There are two possible and equivalent definitions for the Manchester Code, as shown in the figure below:

  1. In the code definition according to GE Thomas, a falling edge means a logical one, a rising edge a logical zero. This definition is also known as Biphase-L or Manchester-II .
  2. In the code definition according to IEEE 802.4, as used for 10 Mbit / s Ethernet, a falling edge means a logical zero and a rising edge means a logical one.

In any case there is at least one edge per bit from which the clock signal can be derived. The Manchester code is self-synchronizing and independent of the DC voltage level. In order to inform the receiver how a logical one is coded in the signal, a header ( preamble ) is sent at the beginning of a data transmission .

advantages

An essential property of this line code is that the resulting signal is free of DC components. This means that the DC voltage component is exactly zero. It is therefore possible to transmit the signal sequence via pulse transformers with galvanic isolation, for example .

Another advantage is that, as described above, the clock signal can be derived from the code itself. An additional clock generator is not required.

disadvantage

A disadvantage of Manchester coding is that the required bandwidth for data transmission is twice as high as for simple binary coding (e.g. Non Return to Zero , NRZ). The reason for this is that two code bits are required to encode a user data bit. It is a 1B2B coding.

The bit rate (user data) (in the case of a two-valued signal) is therefore only half as high as the baud rate (symbol rate in the transmission).

additional

In practice, the coding and decoding can be carried out by an inverted XOR link between the carrier and the useful signal.

In addition to the Manchester code, there is also the differential Manchester code . In this case, in contrast to Manchester coding, a phase change takes place for a specific bit, which is usually logical one. With a logical zero, there is no phase change. As a result, the fixed assignment between the direction of the edge change and the logical signal status is lost and the information can be interpreted correctly even with an inverted signal.

Manchester code is used, for example, for AS-Interface and 10 Mbit / s Ethernet according to the IEEE 802.3 standard; Differential Manchester coding, for example with Token Ring . Furthermore, the Manchester code is used in the watermark of the euro banknotes as a security feature .

etymology

The process was originally developed at the University of Manchester in connection with rapidly rotating plates.

Individual evidence

  1. ↑ http://www.online.uni-marburg.de/hrz/chronik/quellen/sperry-univac-magnetbandsystem-uniservo-30-1983.pdf
  2. ↑ Data Coding and Error Checking Techniques . (PDF; 4.91 MB)
  3. ↑ http://www.roboternetz.de/wissen/index.php/Manchester-_Codierung

Web links