Multiple floppies in one system?

From: John Chris Wren <jcwren_at_jcwren.com>
Date: Wed Feb 27 15:14:19 2002

        Option #2 is a better way. Leaving an unpowered drive on a drive chain can
create problems, since the divers are not properly tri-stated, or drive high
for open collector lines.

        I'd use a double pole double throw switch. Run the two drive select lines
to the center contacts, and top two pins to the two drives you're most
likely to use at one time, and the bottom two pins to the other pair that
are most likely to be used together. Note that you'll want to keep these
runs as short as possible. The drive select lines are, IIRC, the middle two
wires of the group of four that's typically twisted in a floppy cable.

        The original IBM AT (and possible XT, I'm not sure) were the instigators in
using the twisted cables. It was cheaper to have a Tiawanese cable house
make the cables than it was to address the issue of "well, if you want drive
A:, you have to unjumper DS1 and move it to DS0. But drive B: has to have
DS1 jumpered." From a customer support standpoint, and being able to send
drives out, it meant they could pre-configure the jumper for DS1, and be
done with it.

        There are some other approaches: There were some after market cards that
support 4 or 8 drives (8 drives as two controllers, which took extra
software). IIRC, the BIOS actually has support for 4 drives, but it's the
drive select lines that become the issue. I don't have my Book O' Magic
Registers handy, but I think IBM left out the other control bit in the drive
control register. It may have only been some old 486 and lower motherboards
that supported 4 drives, by making the extra drive select bit available in
another register. Once manufacturers realized that no one ('no one' being
all but the 4 or 5 people a year) wanted 4 floppies in the system, support
for it was dropped.

        If you ever only need one drive at a time to be active, you could use a
rotary switch, and switch the drive select line between any of the four
drives. Note that putting more than 4 drives on a chain isn't a good idea,
since the bus drivers start getting near their loading limits. Those extra
signals, even tri-stated, present a bus load, and add capacitance to the
signals.

        What would be slick is to setup some kind of "fast swap" bay, where you
plug the drive you want into a carrier, much like these removable IDE
drives. Sadly, I doubt any such thing exists for floppies, since for all
practical purposes (from a consumer standpoint), floppies are nearing the
end of their life span. What with bootable CDs, CD-R, CD-RW, DVD, and the
incredibly small capacity of a floppy, I predict they'll be a rare item in
10 years, and only used to support the old collectables. Just try and find
5.25" low density disks anymore.

        --John


> -----Original Message-----
> From: owner-classiccmp_at_classiccmp.org
> [mailto:owner-classiccmp_at_classiccmp.org]On Behalf Of David C. Jenner
> Sent: Wednesday, February 27, 2002 15:22 PM
> To: Classic Computer Mailing List
> Subject: Multiple floppies in one system?
>
>
> 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 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.
>
> 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.
> 2) Plug all the drives into one control cable and select the drives by
> switching a control line on the control cable.
>
> Any comments or suggestions would be appreciated.
>
> Thanks,
> Dave
> --
> David C. Jenner
> djenner_at_earthlink.net
>
Received on Wed Feb 27 2002 - 15:14:19 GMT