On Wed, 27 Feb 2002, David C. Jenner wrote:
> I'm putting together an old 486 system, one of whose main purposes will be to
> read/write floppies of several formats. Like 5.25" 360K, 720K, 1.2M, RX50, etc.,
> and 3.5" 720K and 1.44M. I also want to be able to copy RX50s disk to disk on
> the appropriate drive type.
> So I figure it'll take 4 drives (or maybe 3 drives where one is a 3.5"/5.25"
> combo). All of these would be in one enclosure.
The 3.5" 720K and 1.4M can be one drive. There are no significant
problems with reading nor writing 720K in a 1.4M drive.
The 5.25" 720K can be done with the 1.2M drive.
The 360K can be done, with complications and a few minor problems, with
the 1.2M drive. Don't. Use a 360K drive.
There exist controllers that handle four drives. In fact, that was
STANDARD on PC (5150) and XT. AT controllers (that handle
"high" density) that handle four drives are kinda scarce, but do exist.
The "Compaticard" formerly from MicroSolutions is one of the favorite
ones.
There also exist controllers (inc the Compaticard) that can be mapped to
an alternate address, thus permitting using two controllers. But most
programs have issues with addressing a secondary controller.
> The question is, how to do this without having to unplug/plug drives every
> time you want to make a configuration change? It would be nice to just be
> able to throw a switch or two to re-configure. Not having to get into the
> enclosure would be nice, too.
No problem.
> I've thought of two ways to do it, but I'm not sure they'll work, or if there
> might not be a better way.
> 1) Plug all the drives into one control cable and switch on/off the power
> to the drives to select them.
NO! power to the drive is NOT an appropriate way to switch between
multiple drives on the same signal cable!
> 2) Plug all the drives into one control cable and select the drives by
> switching a control line on the control cable.
Assuming that you are, indeed, talking about PC style controllers and
cables (We don't need to be told that that cabling is "non-standard"!),
then you just need to switch pin 12 (at the drive). If you put multiple
drives on a PC style cable, drive numbering is handled by whether the
drive is before or after the twist. ALL drives before the twist will be
"B:", and all drives after the twist will be "A:".
If more than one drive is enabled as a given position, then neither will
work. Therefore, within either of the two drive letter positions, you
want to have pin 12 connect to only one drive at a time. That could be
done by a simple switch at each drive that disconnects pin 12, and then it
is up to you to turn on one and only one drive.
Or you could use a SPDT switch to select between two drives at the same
address. (or a SP25T if you have a lot of drive :-)
Some people maintain that you must also switch the "Motor ON" signal.
That may be true, but I have gotten away with a lot of drive on a cable
and only switched the Drive Select (pin 12) line.
For some formats (including some TRS-80 ones), it can also be helpful to
be able to interrupt the Index signal from the drive, since some fomrats
start their first sector too soon after the index pulse.
--
Fred Cisin cisin_at_xenosoft.com
XenoSoft http://www.xenosoft.com
PO Box 1236 (510) 558-9366
Berkeley, CA 94701-1236
NOTE: My ISP is having difficulties. If you have problems reaching me at
this e-mail address, you can leave a message at:
fcisin_at_merritt.edu
Received on Wed Feb 27 2002 - 15:16:24 GMT