Dec 092017
A hardware approach to installing 3 floppy drives on an AT.
File 3FLOP_AT.ZIP from The Programmer’s Corner in
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A hardware approach to installing 3 floppy drives on an AT.
File Name File Size Zip Size Zip Type
3FLOP_AT.TXT 5967 2612 deflated
FORMAT.BAT 2078 518 deflated
GETCMD.COM 1970 1440 deflated

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Contents of the 3FLOP_AT.TXT file

(without buying another controller card)
By: Bob Sully
(CIS ID 73545,232)

DISCLAIMER: This method is ADMITTEDLY a kluge. It's basically a home-
brewed solution to the problem of running 1.2MB, 360K, and 720K/1.44MB floppy
drives in a machine (an AST-286) with the standard 2-floppy/2-HD Western
Digital controller card and only ONE remaining expansion slot (six being full).
It IS possible to run three (or four) floppies in such a machine concurrently
with some of the new FDC cards now available. I simply didn't want to spend
the money and lose my last slot!
This method allows three drives to be used in the same machine, but only two
are usable AT THE SAME TIME.
Please, don't sue me if this doesn't work in your machine..I can't guarantee
it to work in any other configurations. The system is as follows: AST Premium
286 under AST's MS-DOS 3.3, running TEAC 1.2MB and 360K 5.25" and Sony 720K/
1.44MB 3.5" floppy drives, one inboard 72MB hard disk and outboard 20MB HD.
2M of RAM on the FastRam card and 3MB extended memory on a "clone" multi-
function card (all purchased at between $2 and $2.95/chip!).

Necessary hardware:

(1) 3.5" floppy drive. If you're going with a 720/1.44, get one with the
auto-sensing media switch (automatically switches between 720K and
1.44MB disk formats by checking for the hole in the casing on the high-
density media). The Sony drive is so equipped.

(2) Micro toggle switch (SPDT); cost is about $2.50 at most electronics

(3) 34-line card-edge connector.

(4) 26-gauge wire.

(5) Soldering iron, X-Acto knife (or equivalent), wire cutter/stripper,
electric drill, vise (PanaVise type optimal).


(1) Make sure that your floppy-drive cable is long enough to support three
drives. If not, get one that is. Mount the 34-line card-edge connector
onto the cable with a PanaVise or similar instrument, leaving enough
room between connectors for all three floppies to be properly mounted,
and enough cable to reach the floppy-disk controller card.

(2) CUT line 12 (drive select line for physical drive 1 (B:) between
the new card-edge connector (the first one on the cable when starting
from the controller end). Solder a length of 26-gauge wire to the
controller end of this line. Solder the other end of the 26-gauge wire
to the middle connector (common) of the SPDT switch.

(3) CUT line 12 again between the first and second card-edge connectors
(from the controller end - NOT between drives A: and B:!). Make sure
that the cut ends do not contact each other (otherwise, both the 2nd and
3rd drives would be selected at once!).

(4) Solder a length of 26-gauge wire to EITHER each of the cut ends
of line 12 from Step (3) OR to finger number 12 on each of the two drives
to be switched (the fingers are numbered 2,4,6,8,10,12....on the top of
the card edge of most floppy drive; usually they are marked). If you
decide to use the card edge, solder as close to the front (towards the
front of the drive) as possible, to allow the card-edge connector to make
good contact with the card edge. Solder the other end of each wire to
one of the two remaining connectors on the SPDT switch.

(5) Mount the switch. Most of the 3.5" floppy drives have plenty of room
next to the opening on the front panel. Mine is just to the right of the
disk-access slot. My drives are mounted with the 1.2M as A: on the top,
the 360K in the middle, and the 3.5" on the bottom. The switch in the
"up" position selects the 360K, "down" selects the 3.5" drive.

(6) Re-mount the floppy drives in the computer, and hook up the cable.

(7) Here's where it gets a bit tricky. This set-up works fine on an AST
Premium 286, but in other machines there may be some problems. My
machine's BIOS is set up to handle 720K/1.44 MB floppy drives. I have
mine configured with A: as a 1.2M 5.25", B: as a 1.44M 3.5". I use
DRIVER.SYS to create a second logical device on physical drive 1; its
designator turns out to be G: in my case. This line goes into my
When I boot up, the switch is in the "down" position, so the system
sees the drives as they are configured under SETUP. When I need to
use the 360K drive, I flip the switch up and refer to it as drive G:.
So far, under AST's MS-DOS 3.3, I've had NO problems reading, writing,
or formatting any of the three drives; I do use a neat little utility
called QDR (QuickDiskette Reformatter, available in the public domain)
to format Drive G:; PCTools will also format any of the three drives.
You MUST have a version of DOS which supports all three drive types,
and probably a BIOS which does also, or this probably won't work!

(8) I have included in the .ARC file a little batch file I use to
format floppies. If you look at it you'll notice that I've renamed
FORMAT to TAMROF. This, in addition to the batch file itself, tends
to prevent "accidental" formatting of your hard disk. You will also
need to obtain a copy of Vernon Buerg's QDR.COM for formatting the
third (in this case the 360K) drive. It is in the public domain and
is available on CIS and on most local bulletin boards. I am using
version 3.2.

Happy homebrewing --

 December 9, 2017  Add comments

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