Classic Computer Magazine Archive ANTIC VOL. 8, NO. 5 / SEPTEMBER 1989

Vastly expanded disk capacity for your 8-bit.
Reviewed by Matthew Ratcliff

At one time Atari had intended to produce a 3.5-inch disk drive for their 8-bit computers to replace the aging 1050, a 5.25-inch model. After displaying the new drive at several computer shows, Atari changed its mind and instead produced the XF551, another 5.25-inch disk drive, capable of storing 360K of data per disk in double-sided, double-density format. If Atari had used a 3-5-inch drive, like those found in the Atari 1040ST you could squeeze 720K of data onto a durable little disk that fits neatly in your shirt pocket.

Bob Woolley, a very active Atarian on CompuServe and hardware hacker extraodinaire, decided to see if it was possible to transplant a 720K, 3.5-inch floppy mechanism into an existing XF551 unit. Smart guy that he is, Bob found the task rather simple. All you had to do was remove the 5.25inch mechanism, adapt a 3.5-inch drive to the case, plug in a new cable, and plunk a new EPROM chip into the controller board.

Many other CompuServe subscribers were quite interested in this project, and Mark Elliott of Innovative Concepts (I.C.) wisely made arrangements with Woolley to produce a kit and bring it to market.

The XF35 Kit comes with an upgrade EPROM, a 34-pin cable with connectors, a 4-pin power adapter cable, and a 34-pin male header The actual drive mechanism is not provided, but can be obtained easily through IBM-PC mail order houses. Easy-to-follow instructions lead you through the 18 steps necessary for the replacement of the XF551 5.25-inch drive with a 3.5-inch unit.


Effectively, the 5.25-inch disk drive and controlling ROM are "discarded". An optional set of instructions, involving another 14 steps, shows you how to keep the 5 25-inch drive in place, and add on the 3.5-inch drive, making it possible to select one or the other using a toggle switch.

You will need to purchase the extra parts including an SPDT toggle switch, a DPDT toggle switch, a 5.25-inch disk drive power splitter, a 34-pin 5.25-inch drive connector, and a case for the 3.5-inch disk drive.

I performed the first installation just to verify that the upgrade would work properly. When I was satisfied that I could format, read, and write 3.5-inch disks to a full 720K with SpartaDOS or SpartaDOS X, and also with MYDOS 4.3b, I performed the dual drive upgrade.

A 3.5-inch disk drive can be ordered from just about any PC-compatible mail order outlet. But few of these come with cases. To minimize the "hacking" on a case, cabling and so forth, Bob Woolley recommends a Tandy 720K external disk drive, designed for use with Tandy's 1000 series PC-compatible computers and available and Tandy Computer and Radio Shack stores.

This Tandy drive comes mounted in a nice case that sits neatly on top of the XF551 unit. I found it on sale for $99 (catalog number 25-1061).

To modify the drive, you must first discard the Tandy disk drive interface cable, take apart the case and remove a small circuit board and cable. There will then be a sufficient opening in the back for your drive interface and power cables to come out to the XF551.

An extra connector will have to be added to the flat ribbon cable that comes with the project, to allow the XF551 and Tandy (or similar 3.5-inch drive) to be connected "in parallel." The cable was just long enough to get the job done. The connector required for this task was called a "34-Position Card Edge Connector, Insulation Displacement Type." The cost was $3.95 (catalog number 276-1564A) at Radio Shack.

You will also need a "power splitter" to provide power to both the XF551 and the 3.5-inch units. I happened to have a spare from my last PC upgrade, but they are probably available from Radio Shack. This short cable plugs into the power connector inside the XF551, providing two connectors at the opposite end-one for the 5.25-inch drive and the other for the 3.5-inch.

The instructions specify SPDT and DPDT switches to toggle power (5 and 12 volts) and ROM selections between the two drives-which can only be used one at a time. However, since both switches must be flipped at the same time, a triple-pole, double-throw (TPDT) switch would be more appropriate, as the guide suggests.


The most difficult step in the upgrade is desoldering the circuit board end of the original XF551 disk drive ribbon connector -- 34 pins. I get the best results with a good 30-watt pencil soldering iron and copper braid (also called "solder wick"). A connector is then soldered in its place, making installation and removal of the new interface cable a snap.

For the dual drive upgrade, a second card edge connector had to be added. The sample drawing showed it in the center of the cable, but I found that it fit best closer to the with my Tandy drive.

The power connectors for each drive and ROM selects have to be wired into opposite sides of the toggle switch(es). Care must be taken to get the power and proper drive both on one side of the switch. The switch should only be changed while the main drive power switch is off. With the TPDT switch there is no way to accidentally run a drive with the wrong ROM.

This upgrade went smoothly. Mark Elliot's instructions are completely detailed, and I seldom needed to refer to his schematics. Remember, though, that the upgrade will most certainly void any warranty remaining on your XF551. If you purchase a new Tandy 3.5-inch drive, its warranty is not likely to be valid after cracking its ease, either.

If you are a veteran hardware hacker and have a steady hand you have little to worry about. The upgrade is easily reversed. If you have problems with the 3.5-inch unit, simply replace the 5 25-inch drive. Once you piggyback the ROMs and add the switch, however, the upgrade is basically permanent.


Now, with SpartaDOS 3.2d. SpartaDOS X, or MYDOS 4.36 (and possibly earlier versions of MYDOS). I can create HUGE 720K disks on my 8-bit. The Tandy drive certainly made short work, on a very few disks, of backing up my 20 megabyte FA-ST hard drive. If you don't have a hard drive, these 720K disks may seem like "little hard drives." However, to get the full potential from your new drive, it's a good idea to use SpartaDOS, or some other DOS which allows more than 64 files per disk.

With the dual upgrade, it's easy to toggle back to full 5.25-inch disk compatibility. It's best to have yet another drive in your system, to make transfer between 5.25-inch disks and 3.5-inch disks a bit simpler. A large RAMdisk could also fill the bill.

Bob Wooley has created a patch utility program that will modify SpartaDOS 3.2d to support the XF551's high speed I/O (nearly tripling the data transfer rate between the drive and computer). Transfer rates are greatly increased with the XF35 drive as well. The latest incarnation of SpartaDOS X cartridge, 4.20, also supports high speed I/O on the XF35 upgraded drive.

Don't expect this upgrade to enable you to read and write Atari ST diskettes. The disks may be the same size, but the formats used are different. It may be possible to come up with a program that will write in ST format, but no one has yet tackled the job of writing the complicated software needed.

So you can't afford a hard drive, but want a lot of storage? If you aren't afraid of a little hardware backing, then the XF35 Kit may be the perfect solution to your archive needs. You can copy eight single sided, single density Atari DOS 2.0 disks to one 720K 3.5-inch disk -- a great way to clean up your old disks and recycle a lot of old 5 25-inch disks.

XF35 Kit. $37.95 (incl. $3 s&h). Innovative Concepts, 31172 Shawn Drive, Warren, MI 48093. (313) 293-0730.