Multiple floppies in one system?

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Feb 27 16:51:13 2002

see below plz.

Dick

----- Original Message -----
From: "John Chris Wren" <jcwren_at_jcwren.com>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, February 27, 2002 2:14 PM
Subject: RE: Multiple floppies in one system?


> 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.
>
Actually, there are normally no tri-state lines on a FDD, and the OC drivers
are pulled up by the terminations. Now, if the drive that has the
terminations is the one that's powered down, there could be a problem ... that
bugaboo can be solved by having an external terminator at the end of the
cable.
>
> 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 - 16:51:13 GMT