Forty Years of Personal Computing - 5 1/4" WD2797 Disk Controller

WD2797 controller card for 5 1/4" drives

To work on OS-9, I borrowed some 5 1/4" drives, and used the SWTPc DC-2 controller. This allowed me to boot up OS-9. Single-sided, single-density, 40-track diskettes hold about 100 KB -- they were quite limited on space.

Running OS-9 on single-sided, single-density 8" disks, the situation was a little better, as each drive has about 300 KB of storage. But my two-drive system was limited. Plus, I was something of an island. None of my friends using OS-9 had 8" disks, so I couldn't exchange data with them. It was time to consider 5 1/4" drives.

5 1/4" disk drives went through considerable evolution since their 1976 introduction. The early drives were single-sided, single-density with only 35 tracks. By 1987, double-sided, double-density drives sporting 80 tracks were common. These disks could hold about 640 KB, more than twice what my single-sided, single-density 8" drives held. (And more than single-sided, double-density 8" drives could as well)

Disk Controller

In August 1987, I designed a 5 1/4" floppy disk controller. The 5 1/4" controller is very similar to the 8" design, with appropriate changes for the disk interface. 

A MOTOR ON* signal is generated any time the WD2797 is accessed, with a one-shot multivibrator holding that signal for 10 seconds. Another one-shot asserts the READY signal on the WD2797 after a second of MOTOR ON*. 5 1/4" disks always have the heads loaded, so HLD is tied to HLT.

Back side of 5 1/4" controller

Double-density is jumper-selectable to either follow drive select bit 7, or the SSO output. Side selection is controlled by drive select bit 6. Write pre-compensation isn't used, as it was unnecessary for 5 1/4" disks. 

I built the controller the same piece of 0.1" perfboard that originally held the FD1771 disk controller for 8" disks. The board is a little bit smaller than the WD2797 controller for 8" disks, so it appears more densely packed. Wire-wrap techniques are used for the wiring, and a handful of connectors and discrete parts are soldered.

Drives

For initial troubleshooting, I borrowed the two drives and power supply from a Sage II computer from work, which I had to return. I needed my own drives.

How many drives did I need?  I decided three drives would be sufficient -- one boot disk, and two working disks. This would allow me to copy disk to disk, while still having the boot disk with commands in place. (and no crazy disk-swapping for copies like the original Macintosh that had one disk drive!)

I bought two Tandon TM100-4 drives at a local hamfest. These were common surplus from Lanier word processing units at that time. When I went to buy a third drive, I could no longer find any. I ended up with a Mitsubishi M4853 drive. The specs of the drives are virtually identical, except the Mitsubishi is a half-height drive.  

Drive Cabinet

5 1/4" Drive Cabinet

Finding a cabinet to house three drives was a problem. New metal cabinets are very expensive, particularly in larger sizes, and I couldn't find anything suitable on the surplus market. 

September 1987, I built a wooden cabinet to proper dimensions for three TM100-4 drives. I used 1/4" plywood, reinforced at the corners with 1x1/2 strips. The bottom, back, sides and one quarter front panel are all glued together as one unit. The top screws on to the four corner posts. The finished unit is quite sturdy. 

As originally built, the cabinet was plain unfinished plywood. I recently sanded and finished it with a couple of coats of polyurethane.

Inside the box, plenty of room.

Power comes from a 12 volt, 5 amp supply. 5 volts is provided from a single LM7805 regulator mounted to that supply. In retrospect, the LM7805 might be a bit over-taxed. I suspect the drives draw less power than their maximum specifications. Heat is removed from the cabinet by a small (but noisy) muffin fan on the back panel.

A power switch and neon pilot light round out the front panel, giving a clear indication the unit is on.

The controller and drives work great, easily formatting  double-side, double-density disks using 80 tracks. 

Drives & Software

In April of 1989, I revised all the disk drivers to handle double-density, double-sided drives. The BBUG monitor "D" command code was updated to look for double-density sectors, and the boot loader for Flex09 updated to read double-density, double-sided disks.

For OS-9, I modified an existing driver (FD2) for the Processor Technology PT69 to work with my disk controller and created a new boot disk with several drive descriptors. The drivers and descriptors allowed for 40-track disks (which required double-stepping of tracks, and adjusting the track register), and SWTPc format, where track 0 is formatted single-density -- as well as the standard, double-density, double-sided, 80-track format.

I updated the Boot module to handle double-density, double-sided disks and burned a new OS-9 ROM. 

The result is a smart, efficient unit roughly the same size as the SWTPc 6800 Computer System cabinet. The fan is a little noisy, but was typical for the day. 

Future

The Tandon and Mitsubishi drives only require 250 ms to get up to speed after MOTOR ON*. I can shorten the timing on the one-shot driving the READY signal.

If I can manage to find a second Mitsubishi M4853 drive, four drives would fit into the cabinet. I'd need to add a second LM7805 regulator for the 5-volt supply, and split the 5-volt output across two drives for each.

One limitation of the WD2797 is the track to track and head settling time. These drives can move track to track in 3 ms and need 15 ms for the head to settle. The WD2797, using a 1 MHz clock for 5 1/4" drives, can only do 6 ms and 30 ms, respectively.

Western Digital did manufacture another device, the WD1772-00. This was a 28-pin floppy disk controller for 5 1/4" drives that is software compatible with the WD179x and WD279x devices. The WD1772-00 allows faster track to track and head settling times -- up to 2 ms and 15 ms. 

The biggest problem is finding one, as the WD1772-00 wasn't used in a lot of designs, and Western Digital stopped manufacturing them over a decade ago. Might be interesting for a V3 floppy disk controller card.