S-100 SD Systems Versafloppy II

This page last updated May 12 2020 Contents (c) copyright Herb Johnson 2020. Also look at my SD Systems S-100 Web page for more information about SD Systems S-100 cards.

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I have a number of SD Systems Versafloppy II S-100 controller cards, unused with docs and some software. These cards were designed to operate 8-inch or 5.25 inch floppy drives, soft sectored format, single or double density. Their S-100 design is I/O mapped, uses two 8-bit unidirectional data buses and 16 address lines, and requires the older-style "phase 1" clock line. (YOu can invert the "phase 2" line with a simple LS7404 inverter added to the card.) There is no ROM or serial I/O on this card. The VF II was sold in two slightly different versions, which are described on this page.

These Versafloppy II cards were used by SD Systems in their SDOS systems. SDOS is similar but NOT identical to CP/M. Those systems used a ROM to support the SDOS BIOS and SDOS format programs, and CP/M's equivalents. We have SDOS BIOS and ROM sources, the manual, and various other software, all as described below. We offer no warrenties on any of the software below or included with the VF II card. But there is plenty of information here to help you use these cards and software and to write your own code.

On this page:
SD System Versafloppy II cards available
Bill Beech's work in progress
Merle vanDyke's SD Systems software
CP/M for SD Systems I ran in 1990's
Bruce Jones' work on the VF II
John Monahan's SD Systems work
commentary and code by Fred Scipione and Chuck Guzis
8080 code by Denver Hull
8080 code by Kevin Sisson
updates & comments by Jon Chapman
updates & comments by Roger Hanscom
Other SD Systems VF II related files
Versafloppy II and SBC-200 ROMS

On other Web pages is related information:
SD Systems S-100 Web page.
SD System motherboards available
my floppy drives and diskettes section.
tech specifications on floppy drives and media

John Monahan has pointed out the limitations of these (and other) floppy controllers to operate on slower 8080 or Z80 systems. A 2MHz 8080 can't keep up with double-density data. Here's our email conversation and some photos and documents which provide details.

I welcome comments and questions about any of this donated code. I've posted some suggested code and comments on this site with links as above. Read the code and comments before you do any work or ask questions.

To email me or to order follow this link.
To return to my S-100 home page click here.

SD Systems Versafloppy II S-100 hardware and docs


I have a number of unused SD Systems Versafloppy II "REV P" S-100 cards available. Ask about price and availability.

Documentation is below.
Sample code is below
Version and revision information is below
Other Web sites with more is below.

If you can fix or improve or add to the code or docs, I'd appreciate a copy to share with others who have this card. We try to update this page with new information from time to time.

Versafloppy II versions

All versions, phase 1 clock: All the Versafloppy II cards use a phase 1 clock signal which may not be available on all S-100 systems. See this Web page for a fix to use phase 2 instead.

trimpot version: The Versafloppy II card I offer is a marked "REV P" - look at this photo. It has a WD 1795B FDC chip. Also, the upper left corner has a trimpot or trimmer potentiometer, and an 8-pin IC, a LM310AN op amp. Manual revision B and C seems to cover this version. Manual revision A seems to cover this version.

trimmer cap version: An earlier Versafloppy II card (which I don't offer) is marked REV B or C - look at this photo. The upper left corner has a trimmer capacitor, and two IC's below it, one of which is an NE564 phase-locked loop or PPL chip. Thanks to John Monahan for the revision information, a Rev A manual; and Mike Loewen for improved images of Rev K schematics and layouts.

These changes are in the diskette read circuits. Either board appears to work with the same software. Some customers have reported owning these boards with a WD 1791 FDC chip; this chip can be interchanged with the 1795 chip but the two chips function differently, and disks made with one may have errors when read with the other. Refer to the WD FDC documentation and the VF II manuals for more information.

Versafloppy II manuals, controller chip docs

Please note: the VF II manuals have "revision" letters, which do not match the "revision" letters marked on the boards.

The VF II Rev A manual exerpt of 1979 for what I call the "cap" version is at this link This PDF file only has schematics and notes for the board marked "REV C", with the trimmer capacitor as described above. Since the bulk of the Rev A manual is unchanged in Rev B and C, I only provide a summary of differences between the Rev A and Rev B manuals. You will also need the Rev C manual. I don't have a parts list for the "cap" board; and there are some circuit changes. I could use a cleaner schematic too.

The VF II Rev C manual of 1980 for what I call the "pot" version is at this link. This PDF file has schematics, parts list, and hardware descriptions for the board marked "REV P", with the trim potentiometer as described above. The Rev B manual is only slightly different in text, from the Rev C manual. So I've added a note of differences about the Rev B manual, to this copy of the Rev C manual. Rev B uses a FD1791 FDC chip; Rev C uses a 1795B FDC chip; "read the fine manuals" for details and possible upgrading Rev B to Rev C.

1980 update: Inserted into a Rev B manual, I found this Apr 30 1980 memo.. It says that U13, the 74LS124 in the phase-lock loop circuit, had a "defect" found in 60% of tested LS124s. The defect "locks on the wrong edge of RCLK" and LS124's must be hand-selected to avoid that. But the 74LS629 is an alternative part without the defect; but the following changes must be made. 1) replace C33 with 100pf 2) Vary the adjustment to the VCO input voltage, to see that "the frequency vs voltage adjustment is monotonic". 3) "Insure the VCO input voltage at 2MHz is 2.0V +/- .6 V."

The manuals above do NOT include the data sheet for the WD1795 FDC chip. The Western Digital 1791, 1793, 1795 and 1797 datasheet is at this link.

SD Systems Versafloppy II controller software

Many people have developed software for the Verafloppy II, including of course SD Systems. Unfortunately, I'm afraid to say, each collection of sources has its own "dependencies" and "features" which may make it hard to run on any system, or processor, other than the developer's. One limitation (for SD Systems original code and other's) is if the VF II code calls upon ROMS in the SD System SBC-200 CPU board. Another is choice of assembly language - Z80 Zilog, Z80 TDL, 8080 Intel.

All I can do is offer the code as I get it; reference what it does and how. Over time various people have grabbed this code and made some use of it, as I reference below. So I suggest you look at ALL these sets of codes, see if some are updates of others. Then choose what looks best for your set of S-100 boards. Please offer any results back to me or a link to your results. Thanks. - Herb

CP/M for SD Systems I ran in the 1990's

In the 1990's I operated a SD Systems system with VF II and SBC-200 Z80 board (and SD System ROMS). This system ran both SDOS and CP/M 2.2, and supported in principle 8" and 5.25" disks. Here's the CP/M files from that system. There s BIOS and BOOT sources, plus a set of CP/M programs including SYSGEN, plus FORMAT (as a COM file, no source), plus the SD Systems ROMS hex dumps. It was assembled with SLR's Z80 assembler, so I've included that too. For ROM disassemblies, look in the other files below including from "curt". It's possible you could put these files on an 8-inch SSSD disk and run "sysgen" to that disk to recreate the "boot" tracks. Then boot up the resulting disk to regenerate other formats. No promises.

Merle vanDyke SD Systems code

In Aug 2014, Merle VanDyke contacted me. He acquired sets of SD Systems cards - floppy, CPU, RAM - and code to operate them. He modified the code, and has offered it to me without restriction. Here's the code from van Dyke. So this is likely SD System ROM dependent.

Bruce Jones' software for Versafloppy II

Bruce Jones worked in the industry to "adapt DRI OS products for a large number of S-100 and SBC products across the 8-bit, 16-bit and 32-bit range.". In late 2003, he provided me with very informative discussions and code about 8080 and Z80 programming for floppy disk controllers. I've posted much of that discussion on my site as per my links at the top of this page. As of Jan 2004, I have from Bruce a set of working CP/M 2.2 BIOS, FORMAT, and related Z80 code, for the Versafloppy II (and for other SD System cards). His code (Z80 TDL format) supports 8-inch, 5.25 inch, and 3.5 inch drives. (He also includes a copy of the TDL assembler, ZASM, for CP/M.) All his SD systems code, some CP/M programs, and additional docs in WORD and PDF format are available from my Web site for free download.

One customer, Craig Landrum, who bought the card and software on Jan 2005, said (er, at my request): "The sample boot and BIOS code included with the Versafloppy II board were critical to getting my second IMSAI 8080 system up and running. The documentation included for both the board and the 1795 chip was a great reference, and by examining the code while referencing the documentation, I was able to what worked and get the job done quickly." I'll forward any questions to Craig as he has requested.

For download, here is Bruce Jones' source code, docs and manuals:
Bruce Jones's sources and tool programs;
Bruce Jones's sources documents;
manual for WD179X & TMS279X FDC chip
ZASM manual to assemble code. a document & code for SD Systems' VDB 8024 board upgrades by Bruce Jones;
docs & code on Z80 and 8080 operation of Versafloppy II by Bruce Jones;
docs & code for BIOS and FORMAT with the Versafloppy II by Bruce Jones.
Bruce's BIOS, FORMAT, VDB software and docs ZIP file
Bruce's updates and code fixes for his software

Some manuals for the Teac FD235 3.5" floppy are at this link. As of Oct 2007. I have a variety of floppy drive manuals listed at this Web page for sale as photocopies.

Bruce Jones has released all this work "to the public domain", but please contact me if you want to post or redistribute this work so updates can be made available. Also contact me if you have comments or corrections. We offer no warrenties on any of the software included with this card. But since 2006, Bruce has not responded to my inquiries. So I'll simply add new info to this page.

John Monahan's SD Systems work

From SIG/M disk #26, here's JOHN J MONAHAN's 1981 version of an BIOS and FORMAT and BOOT, written in Z80 code.

In 2009, John Monahan has some SD Systems boards, and discussed with me his work on the SD Systems Versafloppy II controller. Here's our email conversation and some photos and documents. He's provided an older revision schematic of the VF II, and dicussion of how he's recently worked on this board and its software. John provided a BIOS source version and his own VF II diagnostic program source. See John's Web site "s100computers.com" for more information.

Raymon Gandia code for Versafloppy II, SD Systems ROM disassembly

have some SD Systems software provided to me in 1998 by Raymond Gandia as follows below.
(I have not heard from Mr. Gandia in some time.) Any ".CPM" files are actually CP/M .COM files.
GANDIA ZIP Ramon Gandia's work on 3.5 inch support. Sysgen, bios, boot. No format.

John Monahan noted, "One neat piece of software in that Gandia folder is the simple Zilog-mnemonic assembler that quickly spits out .COM code. Always wanted something like this!"

Fred Scipione

Fred Scipione read Bruce's Z80 code in Jan 2005 and provided some comments and code. He has "unwound" the tight read/write loop so a slower processor can execute it. Follow the link for details. Send any comments to me and I'll pass them along. LIkewise I got similar code and comments from Chuck Guzis and I've added that to my Web pages. Thanks!

Denver Hull

Denver Hull did a Z80 to 8080 conversion to run on his IMSAI 2MHZ 8080 board, but he had problems making his VF II work. But the same card worked OK in the same system with Jones' Z80 code on a Cromemco ZPU Z80 card. Here's the 8080 code by Denver Hull, with some of his comments about problems. Code includes BIOS, boot, and format. Also, some 8080 memory test programs Denver obtained from other documents.

Kevin Sisson

In May 2007, Kevin Sisson provided some 8080 code, which I've ZIPpped into into this file. Here's Kevin's discussion of it. "I'm sorry it took so long for me to respond but I haven't looked at the code in a year and a half, and had to find all the pieces again. The file '8080-VF2.txt' explains what I did and has the code that I use on my S-100 system for reading in the boot code from a CP/M floppy. It is an ASCII text file. 'putsys.asm' runs from S-100 memory and copies the CP/M code in that memory to tracks 0 and 1 of a floppy (it is my version of SYSGEN). '4bios.asm' is the entire bios section of CP/M, written specifically for my system. Both of the asm files are also in ASCII, but might have some extra LFs or CRs."

Bill Beech's SD VF II code

Bill Beech in late 2014, acquired some VF II cards from me, and is working on a set of CP/M support programs to use it. He won't use the SD Systems CPU or ROMS. He'll try to do some 8085 and Z80 code. Look for Bill Beech's work via this Web page of mine. As of 2019 Bill has a Web page on the VF II itself and, by the way, the SD Systems RAMdisk board.

updates & comments by Jon Chapman

My good friend and developer Jonathan Chapman AKA Systems Glitch, has worked on S-100 systems for several years. IN mid-2019 he's adapted a number of Versafloppy II's to operate on various S-100 Z80 systems. On his glitchwrks Github site for the VF II he shows his CP/M 2.2 and Northstar Horizon (z80) adaptations of Bruce Jones' and Scipions's V2BIOS.ASM work (previous by Bohdan Tashchuk). However, the code is in TDL Z80 syntax, which means you need to run a TDL Z80 "ZASM" assembler, which Jon provides. jon also provides the CP/M 2.2 .COM files (but not the SYS file). Also, no boot, no format - (shrug) go back to Bruce Jones's works and modify accordingly?

updates & comments by Roger Hanscom

In Nov 2018, Roger Hanscom wrote to me, about working on his VF II with code from Bruce Jones and from John Monahan. He found some issues with constants and ports on those works, as he tried to adapt the code to operate 3.5-inch floppy drives. Also, he's looking to work with Gotek floppy drive/disk emulators. So read Roger's notes on the Jones and Monahan code for more details. Roger fixed Monahan's diagnostic routine myvf.z80.

I didn't add Roger's notes to this page until late Dec 2019. When I contacted Roger with the news, he sent me an update on his results. He got a reduced version of Monahan's VF II diagnostic to work with 3.5" floppy drives. Here's his working version, in Zilog Z80 codes. But he could not generate a good CBIOS for the VF II. He went to fast serial for file exchange, and a S-100 based compact-flash file system, which serves his purposes.

Other SD Systems VF II related files

Any ".CPM" files are actually CP/M .COM files.

Versafloppy II and SBC-200 ROMS

The original Versafloppy II worked with the SD Systems SBC-200 CPU/IO board. ROMS on that board support the SD-Systems CP/M and SDOS (copy of CP/M) boot and BIOS and FORMAT programs. So SD code is not very useful without those ROMS and of course that SBC-200 hardware.

A number of WEb sites have archived the SD Systems ROMS. Marcus' "amaus.net" archive of many S-100 boards and software is one such site. Her's his SD Systems archive. I've grabbed the ROMS and a document from there; they may be Roger Hanscom's old ROM Z80 Zilog disassemblies.

More SD Systems original code

SDSOURCE ZIP SD Systems SBC-200 ROM dissassemblies of various sorts. From "Curt" ?
BIOS, ROM monitors, some format code. May be incomplete.
SDHEX ZIP SD System ROM dumps in hex
disassemblies and assemblies Herb Johnson has done. Some 8080 to Z80 translation tools, assembler and disassembler tools too.
RAMDISK ZIP Jade RAMDISK code for SD Systems, Jade "Double D" floppy controller.
HARD ZIP SD Systems hard disk executables
Also see John Monahan's work above.

Other Web sites for VF II docs or software

Rich Cini describes how to "bootstrap" CP/M using a somewhat similar Compupro Disk 1 floppy controller.

John Monahan's Web page on the VF II, a rev A manual, and additional work on it by him;

Check the SIG/M disks at retroarchive.org copy of the Walnut Creek CP/M CD-ROM. Look for "versafloppy" in the disk and file descriptions. I suggest SIG/M disks 7, 26, 42, 172.


I have some SD Systems SDOS disks, but they are 8-inch and I'll have to read them off someday. SDOS was SD System's rewrite of CP/M.

Herb Johnson
New Jersey, USA
follow this link to email @ me

Copyright © 2020 Herb Johnson