Sardis Technologies ST-2900 SASI Hard Disk Drive Adapter

(This page is still under construction.)

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 catch up on my business accounting, 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, although I don't remember this. But for some reason he convinced me to not use it, and to sell it to him instead.

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 the controller to SASI operation. The system was manufactured in December 1985. It has a full height 5" hard drive, but I haven't yet determined the manufacturer or model of this drive. The controller card is an NDC-5007 from National Computer Ltd., 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 my 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 with the 6522/VIA, but it was so much better than using floppy disks. And this system made it possible for me to file my 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 FLEX drivers for it.

I haven't powered this system up for over 22 years, so don't know if the hard drive and controller and power supply, or the SASI interface, still work.

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

(Click on a picture for a larger version.)
Pictures of assembled SASI board (204-611KB .jpg).
Pictures of NCL SS-10 10MB SASI external hard drive (??-??KB .jpg).

ST-2900 SASI Hard Disk Drive Adapter Hardware and Software User's Manual.
(not available yet)
(??MB .pdf)
DSK image (35T/SS/DD CoCo OS-9 format) of source code and binaries of OS-9 Level I drivers for ST-2900 SASI Hard Disk Drive Adapter.
(not available yet)
(??KB .zip)

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

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Last revised 2020-Jul-31 10:44 PDT.
Copyright 2020 by David C. Wiens.

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