Manchester Decoder

[joseph2k]

'Pure' Manchester code can have pathalogical groups of zeros or ones
making decoding impossible. You also need to either use either Async
(start and stop bits), or Sync (leading known sync byte) to be able to
decode reliably.

Luhan

No, there cannot be pathological bit patterns in Manchester encoding. It
can handle over 10,000 consecutive bits of consecutive 1's, 0's,
alternating, or any other pattern. That is why it was chosen for disk
drives over 30 years ago. It got eclipsed by FM. MFM, MMFM, RLL, GZR, and
other techniques about 20 years ago that achieved higher bit rates with
equal flux change rates (especially for the last three).

 

[joseph2k]

What's VHDL? Does it have a VBE ?

...Jim Thompson

Faux ignorance does not really become you.

 

[Luhan]

No, there cannot be pathological bit patterns in Manchester encoding. It
can handle over 10,000 consecutive bits of consecutive 1's, 0's,
alternating, or any other pattern. That is why it was chosen for disk
drives over 30 years ago. It got eclipsed by FM. MFM, MMFM, RLL, GZR, and
other techniques about 20 years ago that achieved higher bit rates with
equal flux change rates (especially for the last three).

Disc drives use special sync bytes, this makes Manchester workable. A
continuous string of all ones or all zeros still looks identical.

Luhan

 

[joseph2k]

In the early days, Tarbel Electronics produced cassette interface
boards for S100 computers. They used a modification of the Manchester
code. 1's produced a full cycle at the clock frequency, 0's produced a
half cycle at half the clock frequency...

Decoding was more deterministic. You got a 'carrier' of all ones
(clock) followed by an initial zero (start bit) which was easily
detected being twice as long (independant of phase). The data was
decoded as async bytes with 2 stop bits (to give your software time for
storage and other overhead).

This was the system I duplicated on my 8080 system. It got one hard
error in the first year of operation using a standard audio cassette
deck.

Luhan

That was/is not Manchester, that was/is FM.

 

[joseph2k]

Disc drives use special sync bytes, this makes Manchester workable. A
continuous string of all ones or all zeros still looks identical.

Luhan

Sync bytes and preambles are not unique to Manchester, besides for short
transmissions (bit sequences) no sync byte is needed or used for Manchester
or FM.

 

[John Larkin]

Disc drives use special sync bytes, this makes Manchester workable. A
continuous string of all ones or all zeros still looks identical.

But it only takes a single 1/0 or 0/1 transition to get it unconfused
forever after.

John