Sardis Technologies ST-2900 SASI Hard Disk Drive Adapter

The ST-2900 SASI Hard Disk Drive Adapter is an expansion board that plugs into the ST-2900 FDC board to provide a SASI interface to an external hard drive. (SASI stands for "Shugart Associates System Interface", and is the precursor to, and is a fully compliant subset of, SCSI-1.)

SASI Adapter features:

In late 1988 I had need of a computer with a hard disk drive to run my business accounting software, as floppy disks no longer had sufficient capacity or speed to handle the amount of data I now needed to process.

One option was to get a hard disk adapter for my CoCo 3. According to one of my friends, I did buy a Burke & Burke hard disk adapter cartridge (probably the CoCo XT (6MB .pdf)), although I don't remember this. But he says that he convinced me to not use it, and to sell it to him instead, for whatever reason?

Another option was to buy an IBM PC clone with a hard drive, but I really didn't want to go to "the dark side" yet. Also, I had already written custom accounting software for the CSG-IMS database that ran on OS-9, but not on MS-DOS. I didn't want to totally rewrite the software for a different database that could run under MS-DOS, then have to migrate all my existing accounting data to it.

Some time before this I had purchased a surplus NCL SS-10 10MB external hard drive for about C$200. (Just a few years earlier a similar drive would have cost over $2000 new!) It came with an 8-bit ISA adapter card to connect it to an IBM PC, as well as an EPROM that could replace the one on the controller card to convert it to SASI operation. The system was manufactured in December 1985. It has a full height 5¼" hard drive, but I haven't yet determined the manufacturer or model of this drive. The SASI to ST506 controller card is an NDC-5007 from National Computer Ltd. (NCL) in Japan, and is similar to the Xebec S1410.

I made the risky decision to design and build a SASI adapter for my ST-2900 and this hard drive, not knowing whether the hard drive still worked, if the replacement EPROM would work, or if I understood the SASI specification well enough to design working hardware and software. (It was also risky because I had a government deadline to meet to catch up on filing several years of delinquent income tax returns, so couldn't afford to waste time.)

In only 10* consecutive days I designed the hardware, built it (with point-to-point wiring on a Radio Shack perf board), wrote the OS-9 Level I drivers and related utilities for it, and got it all working with minimal trouble-shooting. Wow, was I ever relieved! It wasn't very fast because it used programmed I/O through the 6522/VIA, but it was so much better than using floppy disks. And this system made it possible for me to process all the accounting data for several years and file my past due income tax returns before getting into real trouble.

The NDC-5007 controller can format the hard drive with 256-byte or 512-byte sectors. I opted to use 256-byte sectors to simplify the device driver software (so it wouldn't need to do deblocking), but at the cost of losing 5.9% of the maximum capacity. The driver software (see below) is not an example of my best work because it was written in a hurry and never cleaned up later, but it worked.

However, by this time sales of the ST-2900 had dwindled, and I was now busy with the DMC software and documentation, so I never created a PCB layout for it, and didn't put it into production. And I never wrote any FLEX drivers for it.

I also designed a SCSI interface, but never finished building it.

(Click on a picture for a larger version.)

icon to link to photo of top of SASI adapter board icon to link to photo of bottom of SASI adapter board icon to link to photo of angled view of top of SASI adapter board
Pictures of assembled SASI interface board (204-611KB .jpg).
icon to link to photo of front/left/top of outside of NCL SS-10 hard drive unit icon to link to photo of rear/right/top of inside of NCL SS-10 hard drive unit icon to link to photo of rear/left/top of inside of NCL SS-10 hard drive unit
Pictures of NCL SS-10 10MB external hard drive and SASI controller (241-557KB .jpg).

icon to link to SASI adapter schematics, wiring diagram, PCB layout sketch ST-2900 SASI Hard Disk Drive Adapter schematics, wiring diagram, PCB layout sketch.
(24.4MB .zip)
icon to link to NCL NDC100S Technical Reference Manual NCL NDC100S Technical Reference Manual.
(1.4MB .pdf)
icon to link to DSK image file of SASI OS-9 driver source code and binaries DSK image (35T/SS/DD CoCo OS-9 format) of source code and binaries of OS-9/6809 Level I drivers for ST-2900 SASI Hard Disk Drive Adapter.
(15KB .zip)

*Design Notes

Although I say that the hardware and software were designed, built, written, and tested in only 10 days, I had already done some research over the previous several years. For example, BYTE magazine had a 3-part series in 1983 on building a SASI adapter for the S-100 bus:  March, April, May.  And a May 1983 article in Kilobaud described how to build an SS-30 bus interface to a WD 1000 hard drive controller, complete with schematics and an assembler listing of a device driver for OS-9/6809.

In my design folder I also found information from Wellwritten Enterprises for their SASI adapter for the SS-30 bus and a Xebec S1410 controller, complete with schematics (4MB .zip), source code for SFORMAT for FLEX, and a printed assembler listing of the OS-9/6809 device driver. This listing was very heavily marked up by me, so it appears I borrowed lots of its code for my driver.

I didn't receive a manual for the NCL NDC-5007 controller, but was given a photocopy of one for the NDC100S (see scanned copy above). This appears to be a slightly newer controller that supports SCSI instead of only SASI, but is mostly compatible with my NDC-5007.

Although my schematics show an optional parallel printer port as part of the SASI adapter, I never built it. I also had ideas on how to add a real-time clock (RTC) to this adapter, but never built it either.


As sales of my ST-2900 board sets and DMC floppy disk controller cartridges dried up, I had less need to use my custom accounting software, so used this hard drive less and less. Then in 1993 I wrote a simpler accounting program that ran under MS-DOS on my 286 computer. When I moved in September 1998 I put this hard drive into storage, until ...

I haven't powered this system up for over 23 years, so don't know if the hard drive and controller and power supply, or the SASI interface, still work. Stay tuned for future progress reports as I attempt to get this system up and running again and try to download my old accounting data from the hard drive.

"FLEX" was a trademark of Technical System Consultants (TSC).
"OS-9" is a registered trademark of Microware LP.

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Last revised 2022-May-28 18:12 PDT.
Copyright 2018-2022 David C. Wiens.

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