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Author Topic: Creating EXDOS card clone (Read 25713 times)

Offline Zozosoft

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Re: Creating EXDOS card clone
« Reply #180 on: 2019.June.06. 10:58:26 »
Same COPY with the bad chip. There is some extra head movement. The EXDOS retry mechanism handled it, no error, just about 15-16 secs longer the operation than the normal.
Update: it is only with EXDOS 3, with EXDOS 1.x about 35 Not read question during the COPY :oops:

Offline Zozosoft

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Re: Creating EXDOS card clone
« Reply #181 on: 2019.June.06. 13:22:15 »
The defective VL works properly in the Timex FDD3000 disk drive.
In the original, WD1770 was used there.
Perhaps the faulty VL have a problem with stabilizing parameters when working with shorter access times ?
Maybe your FAFO works a little slower than the original FORMAT ?
I think I found the problem:
The Seek command don't reliable these faulty chips. Sometimes execute Restore instead of Seek (only one bit difference in command code!)
My FAFO only using Step In/Out commands, when Seek needed it is done by steps cycle.
EXDOS FORMAT use DISKIO calls which is use a general head positioning routine. It is don't care about if only one track step needed, always using the Seek command.

Probably that Timex drives also only use Step commands. There is disassembly of their ROM?

If my theory right then the faulty chips also will works with the Hungarian SpeccyDOS disk interface (for a ZX Spectrum), least with 4S ROM (it is developed for a old slow drives, where the 1772 step rates too fast. This ROM using Step cycles with a CPU delays for simulate the different step rates from 2 to 30ms).

Offline pear

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Re: Creating EXDOS card clone
« Reply #182 on: 2019.June.10. 06:14:01 »
Probably that Timex drives also only use Step commands. There is disassembly of their ROM?
It looks like it is :)
Code: ASM
  1. ;==================================================================================================
  2. ; L29b0 Format next track (step in, format)
  3. ;
  4. ;==================================================================================================
  5. L29b0:  ld   a,$01
  6.         ld   ($26a1),a
  7.         ei  
  8.         ld   a,$58
  9.         out  ($c0),a    ; command $58 - step-in, update track register, spin up
  10.         halt
  11.         halt
  12.         halt            ; wait 3 tracks
  13.  

and then $fe - write track, no spinup, 30ms delay, no precomp