Digital Systems Inc. floppy controllers

This Web page last updated June 20 2019. copyright Herb Johnson (C) 2019. . This page covers Digital Systems Inc. floppy disk controller hardware which I own. DSI began in 1975 to produce one of the first CP/M-supported floppy controllers. Some history on Digital Systems Inc. is on another Web page. a review of DS controller hardware and software owned by Jeff Shook is on another Web page. - Herb Johnson

DSI 3.1 floppy controller system - 2013


In Oct 2013, Jeff Galinat contacted me to say: "I picked this Digital Systems floppy disk system up yesterday with an IMSAI machine I purchased. I was mainly interested in its enclosure & power supply to refurbish, repaint & display with the IMSAI as it is basically the same size case. I was going to gut it when I realized that it may have some historical value after I opened it up. I realized that I was looking at a controller that I was not familiar with. Reviewing the information on your site , I realized that I have a Digital Systems floppy disk system with FDC 3.1 controller and HB-14 S-100 bus interface. The drive cabinet is scratched & nicked up but is complete, there is some pitting on the aluminum of the Shugart Single sided drives, but overall it looks in pretty good shape. The FDC 3.1 controller and the HB-14 bus interface are clean.... Would you be interested in a trade for a [drive in a powered cabinent and other considerations] ?" We came to a deal.

I inspected photos of the DSI drive system. The drive controller says "Digital Systems FDC 3.1" and is ink-stamped "227" in black, probably a serial number. There's also an inspection stamp on the back "JW 20". The DSI nameplate on back is model "FDS" serial "8248A" and "115V 60Hz 300W". The S-100 interface board says "Digital Systems Bus Interface HB-1.4" and is markered in black "569".


This photo shows the FDS controller mounted in the cabinet, with the drives underneath. The drives are likely Shugart 800's. A closeup of the controller card is a bit hard to read against the translucent PC board. But careful reading of the chips, written on a high-contast image of the board, provides an IC layout of the controller.


Likewise, here's an image of the Altair/IMSAI interface HB-1.4 board. Again, a high contrast image and hand-copy of IC's provides the IC layout of the interface.

Prior history

Jeff said the IMSAI and DSI drives came from a recycling dealer, who obtained no diskettes or docs. "[The IMSAI] didn't have any disks with it. It had 3 16k IMS static ram cards in it, an IMSAI SIO with 2 ports & cables and the original MPU-A 8080 processor board with a purple gold capped NS 8080 CPU installed; and the Digital Systems HB-14 S-100 interface board. This was a find for me as I now have a good collection of IMSAI hardware but I was lacking the original [IMSAI] CPU card."

DSI 3.3A floppy controller - 2016

[DS FDS 3.3A]

In Dec 2016, I acquired a Digital Systems floppy controller card, from a DS FDS system. I was not able to obtain the drive chassis or drives, the owner did not have the interface card. I was able to obtain the card, directly from the original chassis.

Here's the Digital Systems drive chassis. On the back is the Computerland sticker and the Digital System sticker. It's identified by the DS sticker as FDS and serial number 32691. It may be set at 225V 60 Hz, of course that's the power supply in the chassis. The ComputerLand store was in Austin TX.

With the chassis top removed, the controller card is exposed. On the board, one can see this is a FDC 3.3A PC board layout. There's a black-ink stamped number 30150. Written in marker may be "Lw18" or "Tw18".

Inspection of the IC's, all socketed, finds most date codes are in the first half of 1977, through the 33rd week of 1977. Some chips are later, into early 1978 - one of the 74S472 PROMS is dated the 10th week of 1978.

The back of the board is clean and shows some wiring. Here's detail of the wiring, thre wires added.

DSI Altair/IMSAI bus interface

I acquired this Digital Systems card in Dec 2010, after seeing it on eBay as part of an IMSAI to be sold from the USA in early October. The seller forwarded my interest to the buyer, who was outside the USA. The new owner was only interested in the IMSAI itself and we negotated a trade for another S-100 card (a Cromemco floppy controller). This card has chips dated from 1976, 1977 and 1978.

I acquired a similar card, at the 2016 VCF-East convention, as a donation. Thank you.

Digital Systems FDC and write-protect

The FDC-1 boards, have a means of write-protecting drives with a jumper. How that's done, depends on the FDC version. In June 2019, I was asked by Timothy Colegrove how he could write-enable his just-acquired IMSAI and DS drive/controller system. He was assisted by several other people including Jeff Shook and Larry Greene. So, I dug into my available boards (rev 3.31 controller, rev 1.4 bus-interface) and various docs (described below). Here's the tedious details of my efforts and results. One has to look hard at pin 40 of the floppy-drive side - that's the "write gate" to the drives! - and see what circuits interrupt that signal. I made those determinations, and show the work - so you can do the same! - Herb

Copyright © 2019 Herb Johnson

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