[Coco] some thoughts about reading coco disks on a PC

[Torsten Dittel]

I just figured out how to grab coco disks on < W2K systems with a
software you can find here:

http://www.shlock.co.uk/Utils/OmniDisk.zip

I've been hoping to have more success with this software than with
RETRIEVE.EXE, but it didn't work neither with all those disks (about
20%) having a problem reading sector 1 (at least on the first tracks all
sectors but sector 1 are readable). The command line

omnidisk drive1 dblstepon rate1 mfmon cyls35 minsect1 maxsect18
interleave4 usemanual openoutDISKNAME.DSK readside0 close

will read side 0 of a standard 35 track CoCo disk in drive 1 into an
image file named DISKNAME.DSK which can be used in the JV emulators (you
can make a batch file from it by putting this line in a file named
"RETRIEVE.BAT", replacing "DISKNAME.DSK" by "%1". It can be called like
"retrieve DISKNAME.DSK").

It looks like Omnidisk doesn't support kind of a "Read Track" command
but is trying to read sector by sector. I wonder if this is a limitation
of the PC's floppy controller. Any ideas?

I can't find my documentation on th CoCo's low level format but from the
DECB manual I got:

Tracks
======

Bytes		| Contents
----------------+--------------
0-31		| System controls
32-6115		| Sectors
6166-6249	| System controls (might vary because of speed variations)

and for each

Sector
======

Bytes		| Contents
----------------+--------------
0-55		| System controls
56-311		| Data
312-337		| System controls (might vary because of speed variations)

with the control bytes:

0-7	$00
8-10	$F5
11	$FE
12	track number
13	$00
14	sector number
15	$01
16-17	CRC
18-39	$4E
40-51	$00
52-54	$F5
55	$FB
312-313	CRC
314-337	$4E

What's so weird about this format? Why can't the PC-controller read that
one only on some disks? Any other documntation available on this? I
forgot all this stuff... :-(

Regards,
Torsten