Ok, I think we are getting confused here.  Manchester encoding is not
based on pulse width, but on the direction of the level transition in
the middle of each bit.  There are synchronous schemes that do depend
on pulse width, and which are also fairly immune to speed mismatches
between encoder and decoder, but they are not Manchester.

I also remember another type of Manchester encoding (a quick search of
the web reveals this is called "differential Manchester") in which one
logic level is represented by a level change at the beginning of a bit
boundary followed by an opposite level change in either direction at
the middle of the bit, and the other logic level is coded a NO level
change at the beginning of the bit, but again, a change in either
direction in the middle of the bit.  Effectively, the only real
difference between regular and differential Manchester is the location
of the "intelligence" in the bit stream.  With one it is at the start
of the bit boundary, with the other it is in the middle.

CIAO - Martin.


On Fri, 9 Jan 1998 13:31:13 -0600, John Payson <supercat@MCS.NET>
wrote:

>> Excuse me?  I thought one major reason that Manchester encoding was
>> (is?) popular is that it is self clocking, that is, it is easy to
>> decode since the clock and the data are "combined".  Manchester is RZ
>> (return to zero) encoding, unlike the familiar serial protocol, which
>> is NRZ (non-return to zero), which means that decoding manchester
>> needs none of the "framing" overhead of the serial protocol, such as
>> start and stop bits.  Neither does it require particularly accurate
>> decoder timing, it can tolerate fairly gross timing discrepancies from
>> "nominal", which would render RS232 useless since an FF byte or a 00
>> byte has no transitions at all, and rely on matched XTAL timing to
>> determine the bit boundaries.
>
>Manchester coding uses two lengths of pulse: long and short.  Within valid
>data, short pulses always occur in pairs.  Consequently, it is fairly easy
>to design a manchester decoder which can adapt relatively quickly to changes
>in data rate, and the "paired short pulse" requirement adds a little bit of
>error detection.  Linear time code readers for videotape can often read data
>written at 2400bps nominal when the tape is moved anywhere from 1/10 to 10x
>normal speed (they can also go both forward and backward--kinda neat!)
>
>In some other long/short coding schemes, the lengths of long/short pulses
>differ in a non-2:1 ratio.  I2of5 barcodes code each decimal digit as
>either 2 wide and 3 narrow bars, or 2 wide and 3 narrow spaces; the wide
>parts are 2.5x as wide as the narrow.    Schemes like Manchester coding
>are easier to encode in hardware than those which use variable or fractional
>width pulses, but decoding of corruptible signals may be more reliable
>with variable-width techniques.


Martin R. Green
elimar@NOSPAMbigfoot.com

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