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Thread: Osborne 1 with ROM v1.5/DD thrashing disk on boot

  1. #11


    Yes, it might work on some drives. It will depend on many factors, especially the alignment of the drive but also the read (data decode) circuitry of the disk controller (external to the FDC - designed by the computer manufacturer, i.e. different for Osborne than for Commodore).

    Looking at the code, I see a potential bug in the code. This might effectively trash the retry "timeouts" such that you'd get stuck in the SENDEN routine and would be continually executing RDRV - which steps in once and back out to track 0.

    The retry counter, RTRY, is used by both SENDEN and RDRV, but SENDEN calls RDRV if an attempt to read the disk fails. You call RDRV and overwrite your retry counter, thus the retry counter in SENDEN will never get to zero and you will be stuck there trying to sense the density (performing an FDC read address command, which requires the track to be formatted and set to the right density).

    If this turns out to be true, then it would explain what you see provided the disk cannot be read in either density on the Osborne drive. It could be this 80-track vs. 40-track issue, but also could be something broken in the data decode circuitry (maybe bad components, or out of tolerance, etc), or something else entirely.

  2. #12


    Quote Originally Posted by durgadas311 View Post
    The retry counter, RTRY, is used by both SENDEN and RDRV, but SENDEN calls RDRV if an attempt to read the disk fails.
    Wow, good catch, hats off! I should have payed attention. Yes, on every fail in SENDEN (sense density) the retry counter will be reset to 10 by RDRV, which itself probably is not failing.

    OK, I will keep looking at how to deal with failing media.

    Thanks for your help!

  3. #13

    Default Success!

    To wrap up the thread: I was able to revive both drives on the system after concentrating on things that could lead to read failures.

    The first and most important part was to dispose of the disks that came with the system as they were infectious, tainting the heads of any drive the were inserted to (see previous message about using them in Commodore 128D drive).

    Next of course was cleaning drive heads with ethanol and q-tips. There were no visible residue collected on q-tip, but from my experience with commodore drive I new that was important.

    Next was correcting rotational speed of the drives. I used optical tachometer I had that shines a laser beam and expects a reflected light from a a piece of foil attached to rotating part - a 10 dollar chinese wonder. The measured speed was 294 rpm, which appeared within the tolerance, but after trimming it up to 303 rpm one drive magically returned to life and booted from floppy disk I created on PC from downloaded image using IMD program.

    The other drive did not respond positively to described treatment and required closer investigation with oscilloscope. It turned out it was not producing any read data on its output. Tracing the data signal from input amplifier backward revealed that the signal disappears after entering U13/2 chip (CD4013 D-type flip-flop). Apparently the part was toasted, since replacing it returned the drive to full life.

    In the end I would like to thank again durgadas311 for helping me moving in right direction towards fixing the system

  4. #14


    You're very welcome. You did an outstanding job of tracking down all those faults with the drives. Well done!


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