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Differentiating between Manchester and Biphase

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albert22

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Hi,
I need to check a data stream to know if it is coded in Manchester(BiPhase-L) or BiPhase-M, before decoding it on a microcontroller.
Manchester codes a 1 by a high to low transition, and a 0 by a low to high transition (transitions are in the middle of a bit time)
BiPhase-M codes a 1 by a transition in the middle of a bit time and a 0 by no transition. (There is allways a transition at the start of a bit time).
Although the two methods are diferent, I cannot see the diference between them at the receiver end, to apply the correct decoder.
I would appreciate any ideas.
Regards
K
 

Hi albert22,

To decode Manchester or any Bi-phase Code, as they self-carry the clock pulses
to sinchronize with, the very first task is to recover the clock to get the reference.
The codes are encoded as a signal with two frequencies, Fclock and 2xFclock.
A positive or negative pulse with the length of one clock period must
be detected for the sinchronization. After that and ONLY after that, the bit stream
can be decoded. Getting the graphical shape of the recovered signal you will know
if it´s Bi-phase-L or Bi-phase-M
A good digital scope + patience + some brain is a must.

Best wishes,

humber555
 

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    albert22

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Humber555
Thanks for your answer. I already did some decoding on the data. Both on the screen of a digital scope and later in a routine. The problem is that both methods give a sequence of Fclock and Fx2clock. When I receive this sequence or see it on a scope I can decode it with both decoders, with different results.
I cannot see what could be used to say it is Biphase L or M. not even graphicaly. Am I missing something?
Brain ? whats that?
salu2
K
 

Hi albert22,

I assume that you have at least a clue of the whole packet you are trying to decode.
I mean, you need to know the headers or the sync characters that usually
is the "preamble" wich is sent prior to the "Data Field" to wake-up the remote receiver.
You need to know if the first bit transmitted/received is the LSB or MSB.
If not, you are in problems.
The method described is one that I used a time ago, but I knew what I was looking for.

Best wishes,

humber555
 

Ok Humber555,
you confirmed my fears. I guess I should go for the plan B now. And try to get some of the data you mentioned. Bye
Best Regards
K
 

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