Reading Polymorphic double density double sided disk.
dkelvey at hotmail.com
Fri Jan 18 20:20:45 CST 2008
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
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?
Climb to the top of the charts! Play the word scramble challenge with star power.
More information about the cctalk