Manchester code
In
Manchester code derives its name from its development at the University of Manchester, where the coding was used for storing data on the magnetic drums of the Manchester Mark 1 computer.
Manchester code was widely used for
Features
Manchester coding is a special case of
The
Limitations
Manchester coding's data rate is only half that of a non-coded signal, which limits its usefulness to systems where bandwidth is not an issue, such as a
Manchester encoding introduces difficult frequency-related problems that make it unsuitable for use at higher data rates.[2][3]
There are more complex codes, such as
Encoding and decoding
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Manchester_encoding_both_conventions.svg/650px-Manchester_encoding_both_conventions.svg.png)
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).[4] 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.,
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.
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
Original data | Clock | Manchester value | ||
---|---|---|---|---|
0 | XOR ⊕ |
0 | = | 0 |
1 | 1 | |||
1 | 0 | 1 | ||
1 | 0 |
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, a1
by high-to-low transition (according to G. E. Thomas's convention – in the IEEE 802.3 convention, the reverse is true).[7] - The transitions which signify
0
or1
occur at the midpoint of a period. - Transitions at the start of a period are overhead and don't signify data.
See also
References
- ^ Savard, John J. G. (2018) [2006]. "Digital Magnetic Tape Recording". quadibloc. Archived from the original on 2 July 2018. Retrieved 16 July 2018.
- ^ a b Oed, Richard (22 April 2022). "Old, but Still Useful: The Manchester Code". DigiKey. Archived from the original on 22 August 2022. Retrieved 2 February 2023.
- Cisco Systems, archived from the originalon 28 December 2018, retrieved 12 September 2017,
Manchester encoding introduces some difficult frequency-related problems that make it unsuitable for use at higher data rates.
- ISBN 0-13-066102-3.
- ISBN 0-13-100681-9.
- ^ Manchester Data Encoding for Radio Communications, retrieved 28 May 2018
- .
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 22 January 2022. (in support of MIL-STD-188).