Reading Polymorphic double density double sided disk.

[dwight elvey]

Hi
 I thought I'd update my progress.
 I'd modified the board that was intended to run 8 inch disk. I had the
binary for the EPROM that ran the 8 inch drive.
 The board required a crystal change, capacitor change in the PLL and
one capacitor in a oneshot.
 I decompiled the code for the 8 inch disk and found the read needed
commands and setup to get it to read the disk.
 One the 5.25 disk, it justt wasn't working. I wasn't sure what the problem
was so I brought out the scope to see if I could look at the data going
into the USART.
 At first I thought it might be a problem of the 5.25s using using a
different sync byte.
 I connected the scope up and wouldn't you know it, the delayed sweep
scales I needed to see the first few bits wasn't working correctly.
 I had to debug the scope first.
 This took some time because like most scopes, the circuits are hard
to access ( unlike older Tek scopes ).
 It turned out to be a CD4001 IC that wasn't working correctly.
 Now back to the controller.
 With the delayed sweep I could now look at the bits entering the
USART.
 It was seeing the same sync bytes as used in the 8 inch drive but
I still couldn't read a sector. Of course the calaculations for a 32 sectored
hard drive would be different than a 10 sectored 5.25. Still I should
be able to read something on the first track.
 It took me some time to figure it out but I believe I know the problems.
 The reason I couldn't read any data is that the 8 inch has sync, sync,
track, track\, sector, sector\, data, crc.
 It seems the 5.25 has sync, sync, sector, sector\, track, track\, data, crc.
 Now I need to blow a new EPROM to fix this. I also need to change how the
tracks are calculated.
 For this, I need a little help. Since the 8 inch drives were 32 sectored,
calculating tracks from a disk offset number was easy. 5.25 disk with
10 sectored will require something different. Is there an easy way to
to separate track from sector using Z80 code?
Dwight

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