Problem reading CP/M floppy on a PC - test-drb@ccmp.vtda.org

16 May 2007

Dear All,

Thanks for your helpful reples.

On 15 May 2007 at 11:45, Chuck Guzis wrote:

...
 It's entirely possible to fit 10x512 sectors per
track using a 765-
type controller, though it's much easier if the IAM is left off and the 
space gained used to expand the inter-sector gaps (you can do this with a 
uPD 2765-type controller or any of the WD 17xx/27xx controllers. 
...
 >I think the Tuscan FDC uses a WD1771 controller...

...
 Nope, the 1771 is restricted to FM only.  Maybe a 179x
or 1770/1772/1773 though.

You're right, it has a FD1791 controller.

...
 It's entirely possible that the drive used to
create the diskettes was 
somewhat out of alignment.  You may have to "unalign" a drive to 
successfully read these.  This is more common than you might think. 
Yes, it could be. I tried a different PC last night, same problem as before. 
Using IMD (Image Disk) utility, and the analyse option, it would show a 
scrolling display with entries like:

0  7  20  0  for track 0, the 20 meaning that 20 sectors were found
up to:
39 5 20 0 for track 39.

Using D to read data, I could read track 0 OK, but with higher tracks 
(especially 10 or higher) then IMD gave errors like:

Read error <1> No Addr Mark 2
Read error <2> No Addr Mark 3

etc up to sector 10.

I came across this article:

http://www.atari-forum.com/viewtopic.php?t=9644&postdays=0&postorde…

(Google for ["read sector" "splice points"])

which makes the valid point that floppies are often formated on one machine, 
and written to on another. As I can read track 0, and all the sector marks 
are found for the other tracks, but I can't read the data part of the 
sectors for tracks 1 to 39, I am guessing that the drive used to write the 
data was different to that used to format the floppy and write the system 
track.  The above page talks about 'splice points' where the new data is 
written over existing data.

(Another interesting glossary was found here: 
http://www.softpres.org/knowledge_base)

...
 You didn't mention if you were using a 48tpi
("360K") drive to read these 
or trying to get by on a 96tpi ("720K" or "1.2MB) drive.  The former will 
usually produce better results. 
I am using a 48tpi drive, an IBM badged Qumetrak 142 with belt drive ("That 
is the third worst drive I've dealt with" - Fred Cisin, Oct 2005), also 
tried a Techmate NPH-502. I have some Tandon TM-100s on various machines, 
I'll try one of these plus a 96tpi drive for luck.

...
 If the first sector on the track is consistently
missing (not always the 
lowest-numbered; some systems skew the tracks for better performance), you 
might be able to recover it by slowing the drive a bit to allow the first 
sector IDAM to move outside of the 765 "blind spot" at the beginning of a 
track (although the 765 formats a track with an IAM at the start, it never 
reads it). 
Seems to be every sector past a certan track. BTW the disk is formated with 
sector skewing, starting from track 0 they are read as:

1 2 3 4 5 6 7 8 9 10
9 10 1 2 3 4 5 6 7 8
7 8 9 10 1 2 3 4 5 6
5 6 7 8 9 10 1 2 3 4
3 4 5 6 7 8 9 10 1 2
1 2 3 4 5 6 7 8 9 10

Luckily I have recently heard from two more Tuscan owners with CP/M boot 
disks, so hopefully they will have more joy.

Regards,
John

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