Semi-Virtual Disk Project

An SVD is basically a device that allows you to upload disk images to on-board SRAM. These disk images are used to simulate a floppy disk to your retro computer. Disk images are simply copies of a disk stored in files on your modern day computer.
The Origins of the Sime-Vurtial Disk
The original idea behind the Semi-Virtual Disk (SVD) was originally invented by Eric Rothfus back in 2003. As Eric stated on his website:

"The SVD was built for fun, and was built with parts that were easy to come by and easy to use. Total parts cost is around $60. Schematics, board layout, and all software can be found here, so feel free to build one yourself. What I recommend, though, is that you just buy one here. I don't intend to make any money off of the SVD; whatever profit beyond the cost of parts will be donated to worthy vintage computer sites and concerns."

Eric's original website supporting the SVD is still up and running at

Eric continued to improve and support the SDV project for several years as time permitted from all his other responsibilities. Around 2008 all improvement activity seemed to stop. The last web page update was on October 2014.

So why am I redoing the SVD?

Well I wanted to use the SVD to support my other vintage computer systems. I tryed to obtain a SVD though Eric's web site but all my emails went unanswered. I also wanted to modifiy the formfactor to match a standard 3.5 inch floppy drive so I could install it like a normal drive in the drive array. I turned out that there where others who also where looking for a SDV type solution for their projects.

The updated hardware design presented here is my contribution to the project and meets my needs.

All the features of the original SVD-II work in the new SVD-III. This is no surprise since only the hardware was updates. No changes to the firmware were made and hardware testing was done using the last SVD release V2.4.
The above schematic shows the enhanced version know as the SVD-III. In addition to optimizing the hardware design from Eric, several enhancements were added.

In the original SVD-II board there existed an annoying power up reset bug found. Sometimes the microprocessor would not reset correctly on power up. You were required to manually press the reset button to force a clean reset. In addition, the lower 8-bit of the SRAM address line counter had no provisions to assure it would reset in a zero state. To remidy this issues, a DS1233 was added to assures that both the microprocessor and 8-bit address line counter would be in a solid reset condition after the power supplies became stable.

For some uses of SVD, line termination resistors were required. To support this need in the original SVD-II, an external 150 ohm resistor pack was placed on adapter board between the SVD-II and the interface cable. On the new SVD-III, a 150 ohm termination SIP resistor pack has been added to the board. This allows the termination resistor pack to be installed as needed right on the SVD-III.

A connector used to supports a FTDI module is used in place of a RS232 interface. Through the FTDI module; you simply plug the SVD-III in to a standard USB port to the supporting computer system, select the correct COM port on the PC software and start uploading. The use of the FTDI means that you can set the baud rate to the maximum that the SVD software can handle without issue. An Arduino FTDI module (since I have link 50 of them still in their unopened original anti-static bags) is used. The board should work with others versions though.

The RAM space has been consolidated in to a signal chip. In addition, the new SRAM chip doubles the disk image memory space. Unfortunately the current version of SVD firmware does not support this larger memory size. It is hope that a future updates might take advantage of this increase capacity.  In the meantime a disk image bank select switch option was added. This bank select switch allows two image banks to be supported.

Testing shows that you can place the SVD-III to use Bank 0 and upload disk images to that location. You then switch to Bank 1 and upload another set of images there. You can then switch between the two Banks. For example you may uploaded HDOS in to Bank 0 and CP/M in to Bank 1. Then you would switch between the two Banks and perform a cold boot on the computer to enter the new operating system environment.

One limitation found using this bank switching is if you upload two disk images into Bank 0 and then upload only a signal disk image in to Bank 1. When you switch back to Bank 0 that should have two disk images but it turns out you lose the second image. Looks like the firmware setups an image counter so if you upload only one image, the SVD firmware will not look for a second image.

A battery backup to maintain the SRAM after a power down was also added. Again the current firmware does not use the SRAM after a reset or power cycle until you upload a new disk image. It is hoped that someone will fix the firmware to take advantage of the battery backed up of SRAM since this would be a really nice feature to have.