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Re: Write Signal on Disk II Question
ferdimh@gmx.de wrote:
Yesterday evening I managed to capture first data.
Since the stepper is was not active, I connected my Oscilloscope to
the data output an moved the head by hand until I found a good signal.
This is what I got (I shortened it)
(Garbage) FF FF FF FF D5 AA 96 FF FE AA AA AA AB AB AB DE AA EB FF FF
FF FF FF FF FF FF FF D5 AA AD 96 96 96 (many of them) B4 B4 B4 D2 D2
D2 (many of them)
End of capture (I captured 500 bytes, which exactly fits on the 80x25
screen).
When I decode the Address field I know I'm on volume 254, track 0,
sector 1. The data has not been changed so there is no sync garbage
before it. But I expected the data to be all zero (96 translated). I
ZAPed it on the Apple II and found there WERE all zeroes.
If I shifted D2 left I get 96, so it was reading out of sync.
Right--you're slipping a bit every once in a while. B4 is also a
96 read three bit times late. D2 is 96 five bit times late.
I hooked the motor to by bench power supply and turned the voltage
down to make it run significantly slower. This time it didn't leave
sync, but sometimes detected 3 zeroes instead of 2. (I got 8B).
Sounds like what one would expect from a slow drive.
Turning up the clock (i.e. doing less delay) gave the same result, but
I checked that the timing was ok by sending various pulses in various
spacings into the parallel port. But I only checked with many zeroes,
I din't test the read timing if a 1 was read. Well, if I misadjusted
(shortened) the delay it works pretty well, though I don't really know
why.
Now I'm able to read these all-96 tracks reliably.
Now the next job is arm movement and data decoding.
The problem is that I don't know neither the exact prenibblizing
scheme nor the usual disk image formats.
Maybe someone could explain them to me...?
Sounds like you still are going to have reliability problems with
small variations in disk speed--which can happen *within* a track.
To improve reliability, you must recover the clock and adjust your
nibble framing *dynamically* during the track, as the Disk ][ Controller
does.
You can do the clock recover and framing after reading, but you will
need a higher sampling frequency and a software algorithm to recover
the framing from the bit stream.
-michael
NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
tool--and it's seriously underused."