Osborne-1 SD format

From: Dave Dunfield <dave04a_at_dunfield.com>
Date: Sat Feb 19 09:37:22 2005

Hi Guys,

Here's a question for the diskette gurus - I'm using my Osborne-1
Single-Density disks as an example, however I have observed this
with a number of different machines.

I would like to backup images of these disks - as they are soft-
sector disks, I was hoping I could use Teledisk.

Teledisk does read the disks, however it reports them as 5 sectors/
track, 1024 byte sectors, which doesn't seem right to me - if you
attempt to recreate a disk from the image, it is unreadable.

I thought Teledisk must be out to lunch, and not really likeing
the fact that it uses an undocumented image format, I've been
meaning to write my own replacement for some time.

This morning I wrote some simple test routines to check it out,
basically, just selecting the drive and data rate, recalibrating
and seeking the head, and issuing the READ-ID command to identity
the sectors and type on a track - with "pc" disks, the results are
correct, and exactly what I expected.

With my Osborne-1 single density disks, I can only read Id's with
MDM (ie: DoubleDensity) selected, and ... I read 5 sectors/track
(numbered 1-5), and they are reported as 1024 byte sectors -
exactly the same results that TeleDisk has been reporting.

In a HD drive, I must select a data rate of 300kbps, and in a DD
drive I must select a data rate of 250kbps in order to be able to
read them at all...

I've tried several systems, two of which have 37C65 controllers
with the dual (9.6 & 16Mhz) crystals, which are believed to properly
support single-density operation.

Can anyone tell me what is going on? Can anyone confirm the actual
diskette format of an Osborne-1 single-density diskette?

Why do they "appear" to read at FM 1024 byte sectors (with correct
1-5 sector numbers/ordering)?

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
Received on Sat Feb 19 2005 - 09:37:22 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:37:39 BST