Technical Articles

8bit-Micro talks to the inventor behind the SVD (Semi- Virtual Disk)


As an online vintage computer magazine, one of our tasks is to bring

together in one location the old and new solutions for the TRS-80

community.  We found Eric J. Rothfus a true electronic guru.  He is

the inventor of the SVD, a disk drive device that allows one to connect

a vintage computer to a modern 32 bit computer.  This is not just

another null-modem interface this is a true hardware solution that truly

makes your vintage computer a member of your local area network. 

We contacted Eric and this is what he had to say about the birth if the


8BM:  The Semi-Virtual Disk is for many one of the most

important developments in the TRS-80 community. As it’s

inventor, what got you started on the project?

Eric Rothfus:  I started collecting old computers in 1998, and I

started with my childhood favorite the TRS-80 Model I. In Austin we’re

fortunate enough to have a Goodwill Computer Store, where, while

browsing for some parts, I came across a working TRS-80. Memories

came flooding back, and I had to buy the thing for $10. Unfortunately,

it really didn’t work to well…but it set the gears turning for me

collecting the old machines.

8BM:  A true computer geek, like myself, collects vintage micros.  What I have learned is that many of the

old computers don’t die, unlike the new hardware found today.  Is this what you have found?

Eric Rothfus: It is important to point out that the distinguishing characteristic of my collection is that everything

_works_. I find it most rewarding to find the old machine and to run it as it used to run. This means using the old

hardware with the old software. So I set about fixing that $10 TRS-80, and 6 years later, I have a collection

consisting of numerous TRS-80’s, Apples, Commodores, Heathkit, S-100 based systems, and even a few older

machines such as the DG Nova…all of which run.

8BM:  Lets touch on the SVD.  Did your passion for your collection influence you to engineer the Semi-

Virtual Disk?

Eric Rothfus: Within a couple years of starting my collection, I reached a nice level of frustration in trying to find the

software to run on the machines.  Sure, there are quite a few emulators out there, and you can run software on them

through up-loaded images…but I want them to run on the existing hard.

I fumbled around with getting a 5 1/2″ disk drive on my “lab” (garage) PC and using the emulators to write the disks,

and met with some success…but I couldn’t quite get a copy of Zaxxon to run…and that pissed me off. But, still, I


It wasn’t until I started working with the Heathkit H8 that my direction became clear ;-). The H8 features a hard-

sectored floppy controller, and there was NO WAY for me to get software for it…besides hit-and-miss on E-bay.

The good ole’ 5 1/2″ disk and emulator solution wasn’t possible for the H8.  So I decided then and there that there

had to be a better (and more fun) way to get the old software onto the old machines. So the idea of the Semi-Virtual

Diskette was born (the SVD).

8BM:  Eric, what’s behind the term Semi-Virtual Disk, we know that virtual is the same as ‘almost’ but


Eric Rothfus: I say “Semi” Virtual, because it isn’t a real Virtual Diskette which would probably be some piece of

software that allows connecting the TRS-80 (and other machines) to the parallel port on the PC or something. In fact

I explored that option and determined that with the floppy interface, it wasn’t really practical/possible for that type of

implementation. So I set out to design a piece of hardware that allowed me to download an image (from something

like Ira’s site) from the PC and run it, or boot from it, without any modification of the PC or the TRS-80. This

because my first rule for SVD design: it had to connect to both the PC and the TRS-80/old-computer without

opening either box.

8BM:  I know when I bought an IDE hard drive for my TRS-80 Model IV, I opted for an external drive

because at that time I didn’t what to crack open the case and make modifications.  I can appreciate you

doing the same with the SVD.

Eric Rothfus: Besides the “no modification” rule, my other guiding light for the device was that it had to be cheap. I

really wanted anyone to be able to build one, so it had to be cheap and built with easy to find parts. Also, it had to

have front-end software that made it easy to use on a PC running either Windows or Linux.

So on a Hawaiian vacation in the summer of 2001, I began designing the SVD. Not knowing if I would actually build

the thing or not, I started scratching out schematics and GUI layouts. Two years later, I shipped the first version of

the SVD to a brave soul named Karl VonWinkle who volunteered to Beta test the thing for me. To this day he gets

each one of the new releases.

8BM:  Two years of research and development!  You put a great deal of time into a device you were not

sure would fly.  I guess you have faith in the vintage computer community.

Eric Rothfus:  Sure, two years is a long design time for anything…and I assure you that the SVD wasn’t THAT

complicated. However, as a hobby I only worked on it sporadically, and mostly during my many trips on airplanes in

my “real” job.

So here we are, nearly 3 years after conception with release 1.5 of the hardware…actually, only three releases ever

made it to PCB state, and version 1.7 of the software.

It supports the TRS-80 Model I, III/IV, and Coco. It also supports the Heathkit H8/H89. In my lab support for the

Northstar MDS controller is being created.

8BM:  What do you have in-store of the SVD, features and other hardware it supports?

Eric Rothfus: Slated for the summer is Apple II support. I think I’m a bit late with the Apple II support…that group of

people seem to REALLY want this type of device. There’s another good example of NO OTHER good way to do it.

The H8 community, though, is the most fun.  A fellow by the name of Jack Rubin is almost single-handedly building

a new H8/H89 community, creating a very active mailing list and on-line archive.  The H8 has a nice fanatic following,

and Jack has been quite helpful in getting the SVD support for the H8 as well as the North Star working.  With luck,

Jack will get something akin to Ira’s site up for H8/89 software.

But the TRS-80 community is the most lucky. With Ira’s site, there are tons and tons of images that make using the

SVD a blast. You can run floppy images and individual programs from ALL over. In fact, Ira’s site was the first

beneficiary of SVD “revenue.” With the “profits” from the first few sales, I made the SVD’s first contribution to Ira’s

site. Over time, I hope to support other vintage software preservation sites.

But the most important thing for me is that with the SVD I can run the old software. Check that, the most important

thing about the SVD is: I can run Zaxxon!