Osborne-1 SD format

From: Dave Dunfield <dave04a_at_dunfield.com>
Date: Mon Feb 21 19:06:01 2005

Hi Dan,

>|With a PC format disk, and most other DD disks, I can simply issue
>|a "read id" with the wrong data type - this times out at the index
>|hole after two revolutions, at which point I set the right data type
>|and perform repeated "read id"s until I see the same id again - on
>|most disks, this quite reliably gives me the sector interleave pattern
>|on the track.
>|With the Cromemco disk, this does not work, as often I won't see all
>|of the sector id's until I have reissued the "read id" many times,
>|often seeing some sectors 30-40 times before seeing one occurance of
>|the "tough" sectors. Clearly even if I do eventually get all the
>|sector Id's, I cannot determine what the interleave is at all..., not
>|to mention that this makes the "analysis stage" a but lengthy.
>Try adding an increasing small delay after the read id you use to
>synchronize with the index hole.

Yeah, I tried that earlier on with mixed results, and didn't seem to
really get anywhere - but I've cleaned up a few other things now, so
I'll revisit it.

>|(read-track seems
>|pretty useless on the 765 series).
>It is. Your best bet would be to get a 1797 family controller.

Believe me, I've thought of that - problem is that I want to be able to
send this to people to make disks - but it looks like the pee-cee controller
(or the design) is just not up to snuff, and all I see a people having
problems - single-density on a pc is very problematic.

I've also toyed with the idea of building a very small embedded controller
with a 1797, few tracks worth of buffer storage, and a reasonably fast serial
link to the PC. This is basically what I am doing now to archive/restore North-
Star disks (which are hard-sectored), except that since the N* controller
is well defined, I can just put my code into any N* system (user supplied his
uart routines).

[or if you wanted to be super-flexible, have just a DSP - but thats a tad more
 software work than I feel like undertaking right now...]

>|It's interesting with the Cromemco CDOS disk - the DD area is formatted
>|to 10 512 byte sectors/track, and in the DD(360k) drive, I can't read
>|them at all - For this test, I pulled the actual Teac drive that I have
>|been using on the System-3 (which reads it fine) - it looks like the PC
>|controller has touble with the tightly spaced sectors.
>It has trouble with various kinds of back-to-back operations, but I
>wouldn't expect that to make the tracks totally unreadable. Something
>else is going on. Is it possible that the BIOS thinks you have an
>80 track drive and is double-stepping? That would let you read track
>0 but no other.

I'm not using BIOS at all - I'm communcating directly with the FDC and
DMA controller. It simply does not see Sector-1 on the SD track, almost
all the time!

>|It (the DD drive) can however read all of the SD sectors in track-0
>|just fine!
>|The HD drive reads the entire disk perfectly, EXCEPT for Sector-1 of
>|Track-0 (the SD track) - it quite reliably refuses to read the first
>|sector of the SD track. The remaining sectors of the SD track and all
>|of the DD tracks read OK.
>Try covering the index hole cutout on the disk or otherwise blocking the
>drive's index sensor. The controller does not like to start a read too
>close to the index hole, but it will mostly work without any index pulses.
>(It won't format or timeout of course.)

Interesting idea - I'll give that a try, at least it might shed some more
light on what exactly is going on.

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
Received on Mon Feb 21 2005 - 19:06:01 GMT

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