TU58 problems

[Jon Elson]

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.

Jon