elson at pico-systems.com
Sat Jan 16 14:27:16 CST 2016
On 01/16/2016 11:44 AM, tony duell wrote:
> So I get that
> a bit should take around 41us. And a bit starts with a rising edge, the position of the falling
> edge (recorded at 1/4 or 3/4 of the bit time) determines whether it's a 0 or 1.
No, not correct for 800 BPI (NRZI). That is somewhat how
1600/3200 PE tape works.
> Well, sometimes that's what I see on the LogicDart. Sometimes I see a 1:1 square wave with
> a period of 40-odd us.
800 BPI (NRZI) only has transitions on a data track when
there is a 1 recorded. There will always be at least one 1
recorded in any byte, as the odd parity track will have a 1
for an all-zero data byte. So, a track that has all 1's in
consecutive bytes will have your ~40us square wave. When
the head is porperly aligned for skew, there will be a skew
test point that should have an approximate square wave.
When the skew is bad, this will have a wave with stepped
sides, showing where individual bits came in with a time skew.
If a track has a run of zeroes, then that data track will
have no transitions in it for that interval.
At the end of the data block, there will be a 2
character-time gap, then the CRC and LRCC will be recorded.
The CRC is computed with a shift register with XOR gates,
the LRCC is produced by just clearing the NRZI register. it
is a longitudinal parity of all the 1's in that data track.
If a track has an odd number of 1's in it (including the
CRC) then there will be a transition in that bit of the LRCC.
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