1.2M HD / 300kbps / single-density
dave06a at dunfield.com
Sun Mar 18 20:36:40 CDT 2007
Today I stumbled across something that may explain why some
people I've corresponded with have had such a hard time
getting single-density to work with disk imageing.
For my own setup, I have a couple of trusty Aopen AP5T
mainboards which do single-density nicely, installed in
cases with an internal 3.5" HD (1.44M) drive as A:, and
an external cable allowing me to connect any of the
following as drive B:
- 5.25" DD 40 track (Teac 55-G)
- 5.25" DD 80 track (Teac 55-F)
- 5.25" HD 80 track (Teac 55-G)
- 8" DS (Qumetrack 242)
I'm also quite a stickler for using the proper drive when
I read/write images - so if it's 5.25" 40 track, the 55-G
gets connected etc. This has worked very well for me.
Today I was setting up another system to do 5.25" disks
only, and I wanted it to be self contained within one case.
To minimize the drives, I decided to modify a Panasonic
JU-475 with a switch on the front bezel to force it to 300
rpm to serve as both the DD/80 and an HD/80 drive.
To verify that the drive was good, I hooked it up and tried
to read/write some disks - to my surprise I could not do
single density (at 300kbos). Tried a couple other HD drives
with the same result. On a hunch, I modded it to 300rpm, and
sure enough, I can read/write single-desnity fine at
300rpm / 250kbps.
(Yeah, I know the SD rate is really only 1/2 - by 250/300kbps,
I am referring to the MFM settings for the AT controller data
rate select register).
To rule out some odd ImageDisk quirk, I tried several versions
of TeleDisk - with the drive at 360rpm and configured as a 1.2M
HD drive, none of them could read a single-density disk either.
So at least for my AP5Ts, it appears that the internal controller
can do single or double density at 250kbps, but only double-
density at the "at compatible" 360rpm rate of 300kbps. My guess
is that the data separator does not work at that rate (works
fine at 500kbps however).
I'm curious to know if this is a charactistic specific to the
machines I am using, or if it is common among PCs that do single-
density to not work at 300kbps. Has anyone here read and/or
created single density 5.25" disk on a 1.2M HD drive spinning
PS: I've added a note to the ImageDisk help file and manual
about this (that you may need a DD drive to do single-density),
and I've also put up a brief page describing the modifications
I made to add the speed switch to the Panasonic drive on my
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
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