Manchester-Code

Manchester code is a line code, which receives the clock signal in the coding. In this case, a bit sequence modulates the binary phase of a clock signal. The Manchester code therefore constitutes a form of digital phase modulation, which is also referred to as phase shift keying.

In other words, bear the edges of the signal based on the clock signal, to receive the information.

General

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

In each 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. To tell the receiver as a logic 1 is coded in the signal, with a header ( preamble) is sent at the beginning of a data transfer.

Benefits

An essential feature of this line code is the DC component of freedom of the resulting signal. This means that the DC voltage component is exactly zero. Therefore, it is possible to transmit the signal sequence for example the pulse transformer with a galvanic separation.

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

Disadvantages

A disadvantage of Manchester encoding is that during the data transmission, the bandwidth required is twice as high as for the simple binary code (for example Non Return to Zero, NRZ). The reason for this is that for the coding of a bit two signals are required. The bit rate (in the case of a bivalent signal ) is thus is thus only half as large as the baud rate.

More

Practically, the encoding and decoding can be performed by an inverted XOR of the carrier with the desired signal.

In addition to the Manchester code, there is the differential Manchester code. In this place, in contrast to Manchester encoding, at a given bit, usually it is logic one, a phase change takes place. When a logic zero, no phase change occurs. This is the fixed assignment between the direction of the edge change and logical signal state is lost and it can thus also with inverted signal the information to be interpreted correctly.

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

Etymology

The method was originally developed at the University of Manchester in the context of rapidly rotating disks.