[Coco] Need help with 3.5" drives and NitrOS-9

[Robert Gault]

If anyone has a 3.5" drive attached to a Coco system please respond to this message.

Currently NitrOS-9 sets all descriptors for 80-track disks as 3.5" types with 
typ=$21. When such a disk is formatted, it is stated to be set to 135tpi.

Now if I try to read such a disk on my Coco3  with either a Tandy controller or 
a Disto SCII it will read correctly. However all writes to such a disk corrupt 
each byte usually forcing the msb high.
If I use dmode to set typ=$20 for such a disk, format makes it a 48tpi disk and 
there are still 80 tracks. This type disk both reads and writes correctly on my 
system.

Clearly something is wrong. It does not seem to be my controllers and I don't 
see how it can be the 3.5" drive. The first question is does anyone else with 
3.5" drives see a similar behavior. The second question is how should this be fixed?
Is a Coco system just too slow to write 135tpi disks? Is perhaps the 3.5" drive 
on my Coco (MITSUMI model D35M3D) not capable of 135tpi writes? But then how can 
it read such a disk?

Apparently 3.5" disks can be formatted at 48tpi for 80tracks "double sided" and 
perhaps they should be for Coco systems. If that is the case, there should be 
some changes to one or all of rbf.d, rb1773desc.asm, and rb1773.asm in NitrOS-9.

Robert