For the last few years I've volunteered at the System Source Computer Museum. I've done a couple of projects for them, some of which I've described in this blog. But this has also exposed me to the world of vintage personal computers. Hobbyists spent incredible amounts of time collecting and restoring computers from the dawn of the PC era.
One of the challenges in the "vintage" computer world is finding replacement parts. Maybe you need to fix a vintage keyboard whose foam components have turned to goo. Or you have a 1980s era PC that requires disks with the ST-506 interface (often called "MFM" disks for the recording format). I have a VAXstation 3100 that requires narrow-bus SCSI disks no larger than 8 GB. An entire cottage industry has evolved to satisfy these needs.
One of the folk I bump into at the museum had a challenge: his wife had some documents on an old, non-functioning computer that she wanted to retrieve. He wondered if he could build something that would let him read data from the MFM-type disk drives in the machine. So he did. Over the years this has evolved into a very popular product that not only will let you read MFM-style disks, but will emulate such a disk drive. This gives vintage computer hobbyists the option to run their vintage computers without having to find working disk drives that haven't been manufactured in decades. Similar products exist to replace older SCSI drives like in my VAXstation.
As I mentioned in my post A look at a vintage PC's disk storage subsystem, there was an evolutionary step between the MFM-type disks and IDE disks: the Enhanced Small Device Interface, or ESDI. This was designed by Maxtor in 1983 and became popular in high-end systems until supplanted by IDE and other technologies in the early to mid 1990s.
There are several computers at the museum which require ESDI disk drives, such as the IBM System/36 5363. Museum volunteers have others, including a DEC MicroVAX II and some Silicon Graphics systems. No emulator existed for these ESDI disk drives. If When the original drives fail these machines would become inoperable.
Having discovered the need, I thought it would be fun to create an ESDI disk emulator. I started down this path in the fall of 2024, gathering documentation, ESDI controllers, and ESDI disk drives. One of the challenges of emulating an ESDI drive is that it is both more complex than the ST-506 (MFM) interface and much higher speed: ESDI tops out at 24 Mbps.
Many years ago I collaborated on a proposal that would have been based on the Xilinx Zynq-7000 system-on-chip, and acquired a Digilent Zybo reference board when it first became available. That proposal didn't come to fruition, but I've always wanted to do something more with the Zynq SoC.
I decided to merge these two intersecting interests into one project: the ESDI-Emu.
At VCF East in April, and at VCF West in August, I showed a technology demonstration of the ESDI-Emu using a prototype of the design. The tech demo showed an IBM PS/2 model 80, which came factory equipped with a Microchannel ESDI controller and an ESDI disk drive, booted and running from the ESDI-Emu prototype. I also showed an IBM PC-AT, equipped with a Western Digital WD1007 controller, booting and running from another ESDI-Emu prototype.
Now I've received the first batch of pre-production ESDI-Emu boards:
This single board will emulate up to two ESDI disk drives. It can also be configured as an ESDI disk controller to clone a functioning ESDI disk drive. User data is stored on a microSD card. It includes an integrated Supercap power backup subsystem to ensure cached data gets written to the microSD card when the user shuts off the system power.
Many ESDI drives required a 5.25" full-height drive bay. The ESDI-Emu card is designed to fit into a 3D-printed carrier, and that carrier can be mounted in a 3.5" half-height drive bay.
For size comparison, here's the ESDI-Emu sitting atop a 5.25" half-height MiniScribe ESDI disk drive:
The board shown is one of the pre-production batch I'm using to qualify the design, prior to a larger but still limited production run.
If you have a need for an ESDI emulator, please comment on this post so that I can gauge how many I should have made.
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