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Adding An External 3.5" Floppy Drive To A Tandy 1000A (HD)
With the increasing popularity of the 3.5" microfloppy disk format, I'd been
searching for an inexpensive way to add this capability to my three year old
and very reliable Tandy 1000A (HD). It would have to be an external drive
since the computer has no room at all left for any internal stuff. It
contains a Memory Plus board (512K) with a piggy-backed RS-232 card, a 286
Express accelerator card, a 20 megabyte hard card whose controller also
controls the original 10 megabyte internal hard drive, the original 5.25"
floppy drive and a Smartwatch. There are plenty of 3.5" drives on the market
but most are intended for internal mounting and don't include cabinets.
Recently Tandy put its external 3.5" drive for the 1000 EX (25-1061) on sale
for $99.95 (originally $279.95). Now this drive was intended for the 1000 EX,
which has a 30 position external drive connector on its rear apron, and it
comes with a special cable which has 30 position card edge connectors on each
end. In addition to data and control signals, power for the drive is supplied
through this cable. Out of the box, the drive isn't directly compatible with
the 1000A, which has no external drive connector. If you remove the cover
from the drive you'll see that there is an adapter card which has the 30
position edge connector for the special Tandy cable. From the card there is a
standard 34 conductor ribbon cable terminated with a standard 34 position
female header connector which plugs into the drive itself. Also from the card
is a 4 conductor cable which plugs into the drive's power connector. This
cable supplies +5v (red) and +12v (purple) to the drive. The two black wires
are both ground connections. To make this drive function with the 1000A it is
necessary to bypass the adapter card and connect directly to the drive itself.
Also, you need to be running MSDOS 3.2 or 3.3 for this to work.
Materials needed:
1 - Model II/12/16 printer cable (26-4401, $29.95)
1 - 34 position female header connector (276-1525, $2.49)
1 - 34 position female card edge connector (276-1564, $2.99)
6 ft of 4 conductor cable (#18 stranded is ok)
Wire Tap-Ins (64-3052, $1.69 for a package of 10)
Note: If you already have some 34 conductor ribbon cable, you don't need that
printer cable; just get one more header connector (276-1525).
Remove the 36 pin printer connector from the printer cable. If you don't
need the full 6 ft length of the cable, you can cut the cable to the desired
length with sharp scissors. Now install a header connector (276-1525) on the
cut end of the cable. This should result in a cable with female header
connectors on each end. About 6 inches from one end, install the 34 position
card edge connector. Now, you're ready to hook up the drive to the computer.
Remove the cover from the computer (which should be turned off, of course).
Next, locate the short ribbon cable connecting the mother board to the back of
the 5.25" floppy drive, unplug and remove it. Plug your new cable into the
mother board and 5.25" drive and feed it through the back slot of the 3.5"
drive. Unplug the cable from the adapter card of the 3.5" drive and plug your
cable in its place. If the colored edge of the cable is to the right as you
face the computer it MUST be plugged into the 3.5" drive so that it is to the
left as you face the 3.5" drive. Next, using the Wire Tap-Ins, connect the 4
conductor power cable from the 5.25" drive's power cable to the corresponding
conductors of the power cable leading from the adapter card to the 3.5" drive
module.
Now, turn on the computer and boot it up. Add the following line to your
CONFIG.SYS file:
DRIVPARM=/D:1 /F:2
This tells MSDOS that you have a 720K 3.5" drive as Drive B:
Reset the computer to read in the new CONFIG.SYS file and you're done.
After testing everything, replace the covers on the computer and 3.5" drive.
If you lay the ribbon cable flat over the power supply cage, the cover will
slip nicely over it. I ran the power cable out through a gap between the
plug-in cards on the computer.
The MSDOS 3.2 FORMAT program will format the 3.5" diskettes properly and
CHKDSK will identify them correctly. DISKTYPE will also identify a formatted
720K 3.5" diskette but tell you that you can't format it on this computer.
This is NOT correct; you most certainly can.
Tom Price - September 24, 1988