The Story of the Sardis Technologies ST-2900 Computer System

(Note -- I am writing the following mostly from memory, with some supporting reference materials, over 35 years after the fact, so hope I got the details correct.)

Why did I design the ST-2900 Computer System? After a few years of using my SwTPc system, I decided I wanted a more modern, much more compact, and more economical 6809 system for myself, that supported 64KB RAM, with I/O and ROM occupying as little as possible of the address map (especially beneficial for OS-9), and could handle double-sided, double-density 5" floppy disk drives. I also thought that such a system would be of interest to other people, and since one like it wasn't available on the market at that time (1982), it could provide a business opportunity for me.

The computer that provided some of the inspiration for the ST-2900 was the low cost Radio Shack Color Computer (CoCo 1) that came out in 1980. Most importantly, it used the same Motorola 6809 microprocessor that I was currently using and really liked. The base system was all on one circuit board, and it used inexpensive DRAM, along with a support chip that made DRAM easy to use.

But there were many things that I didn't like about the CoCo, which is why I never owned a CoCo 1 or 2. It only ran its 6809 processor at 895 kHz -- I wanted a 1.000 MHz system, which would make it easier to calculate execution times of instructions, besides being slightly faster. (This was important for double-density floppy disk I/O.) The CoCo 1 could only display 16 lines of 32 uppercase characters. The RAM used by the video display left less memory for the operating system and user programs. The keyboard left much to be desired. Its only built-in I/O ports were an audio cassette interface, joystick ports, and a serial port that was bit-banged in software, requiring interrupts to be masked while a character was transmitted or received -- not good for multi-tasking under OS-9. Expanding a CoCo system (in 1982) was awkward -- if a person wanted to add "real" serial ports, parallel ports, and a floppy disk controller, they had to buy 3 separate cartridges. But the CoCo only had one expansion slot, so only one of these cartridges could be used at a time -- the Multi-Pak Interface wasn't available for the first few years. Although the new system I was designing would have limited expansion capability (to keep it compact and low cost), it would have all the most important features built-in.

Other computers on the market at that time that may have also inspired me were several S-100 bus Single Board Computers (SBC) such as the SUPER/NET (1982) from Advanced Micro Digital Corp., the Big Board (1980) that many of my fellow computer club members used, the Chandler 6809 CPU (1982), and maybe some G-64 boards from Gespac. There were ads for a 6502 SBC (p.34) and a 6809 SBC (p.62 (p.22 of catalog)) in the November 1982 issue of the Canadian magazine Electronics Today International (ETI). National Semiconductor produced a system of small (3.3" x 1.6") stackable modules with 52 pins called the MA2000 series (pages 12-3 to 12-24) (1982), which didn't use any backplane. There probably also were others that I can't remember. Boards that came out after I had started designing the ST-2900 are the Ampro Little Board Z80 (1983), and especially the Megatel QUARK/100 (1983?) -- but I'm not sure if I saw ads for them before I had already finalized the design of the ST-2900.

I decided to split the new computer system into two smaller boards instead of having a single larger board, to allow various configurations. Putting the 6809, DRAM, EPROM, serial ports, and a counter/timer on the CPU board would allow it to run all by itself for some simple applications. The CPU board could be paired with a custom I/O board for control systems, or the FDC board could be plugged into the CPU board to add a floppy disk controller and parallel ports to make up a complete general purpose computer system.

I had originally hoped to fit the entire system onto two 2-layer boards only 100mm x 100mm (3.9" x 3.9"), but when I realized it wouldn't quite fit, selected 100mm x 160mm (3.9" x 6.3"), which is the standard single Eurocard size. This was fortunately also a good match to the new 3.5" floppy disk drives that were introduced shortly after I decided on this form factor. Because I didn't want a separate backplane to connect the boards, I omitted the Eurocard's DIN 41612 connector, instead using 60-pin (2x30) male and female board-to-board connectors. Connecting the CPU and FDC boards together back-to-back, sandwich-style, would make a very compact system, yet allow easy access to all components.

In November 1982, when I was working on the detailed design of the ST-2900, 32Kx8 static RAM chips were not yet available. Even 8Kx8 SRAM chips were either unavailable or very expensive, and in any case 8 of them (at that time only available in .600" 28-pin packages) would have occupied too much board area. The alternative was to use 8 of the newly available 64Kx1 DRAM chips, which came in compact .300" 16-pin packages, and were more reasonably priced. But these required address multiplexing, periodic refresh, and tricky /RAS and /CAS control signals.

Fortunately Motorola's Synchronous Address Multiplexer (SAM) chip (MC6883/SN74LS783 and later SN74LS785) was already available and used by the Radio Shack Color Computer. It provides DRAM address multiplexing, automatic refresh, and generates the /RAS and /CAS control signals. It also generates the E and Q clocks for the 6809E microprocessor, implements address decoding, and allows switching between ROM/RAM and all-RAM memory maps, all of which would otherwise take many chips to implement. Together with one other chip, it provides three chip selects of 32 bytes each, perfect for the three I/O chips I was planning to use. Most importantly, the I/O is located at $FF00 and above, leaving 63.75KB of contiguous RAM.

I chose not to add the MC6847 VDG chip to generate video, instead planning to connect a separate CRT terminal via a serial port. This would provide a higher quality 24x80 (text-only) display instead of only 16x32, and would eliminate using precious RAM space for a video buffer.

I selected the new Signetics 2681 DUART chip for the serial ports, even though it wasn't software compatible with the Motorola 6850 ACIA typically used by 6809 systems of the day. I chose it because it provided two serial ports in one chip, included a 16-bit counter/timer and several spare input and output lines, and had 4 built-in baud rate generators, all of which helped reduce the chip count of the system. Other benefits were that both receivers were quadruply buffered, and could use hardware RTS/CTS flow control, to minimize receiver overruns at higher baud rates. It even allowed configuring a different baud rate for the transmitter vs. the receiver, and supported fractional stop bits.

But when I tried to obtain some 2681 parts for my prototype I discovered that the chip was so new it was only being sampled to selected customers! (The downside of choosing parts from magazine ads!) Fortunately a fellow computer club member worked for a local company that had already obtained samples. When they received new engineering samples, he let me try the older samples. They proved very difficult to get working -- I don't know whether some of it was due to hardware bugs in these early samples, or incomplete or incorrect information in the data sheets? Eventually with lots of trial and error using different programming sequences, and getting newer samples to try, the 2681 started working.

I selected the MOS Technology 6522 VIA chip instead of the Motorola 6820/6821 PIA chip because it provided two 16-bit timer/counters and an 8-bit shift register (in or out) in addition to the two 8-bit parallel ports (with handshaking lines).

I chose the Standard Microsystems Corp. (SMsC) FDC9229T/BT (p. 497) to partner with the Western Digital 1793 floppy disk controller chip to provide adjustment-free digital data separation and write precompensation for ease of use and reliability for both single- and double-density operation. I designed it to allow four 5" and/or 3" drives to be attached.

By March or April of 1983 I had finished a preliminary design, built a wire-wrapped prototype, and got much of it working. (Unfortunately I can't find any pictures of it.) Because this was my first microcomputer design, I had several experienced electronics engineer friends look at the schematics -- they didn't find anything significantly wrong with the design! It helped that I had spent lots of time carefully studying all the relevant data sheets and application notes from the chip manufacturers, and analyzed the timing of the signals. Work continued on the monitor firmware and a port of the TSC 6809 FLEX operating system.

In September 1983 the design was mature enough that I hired Jon Rescorla of Quantum Designs to produce the PCB artwork for the CPU and FDC boards, since I had no experience (or patience for) creating hand-taped artwork for PCB layouts. (PCB CAD was very expensive in those days, so was used mostly for very complicated boards.) He had just started the layout when I received the '68' Micro Journal magazine issue with Peripheral Technology's ad for their new PT-69 single-board computer. I felt quite discouraged, knowing that their board would be stiff competition for mine, but decided to keep going, as mine was different enough that some people would prefer it.

After receiving a few prototype PCBs and assembling and testing them, I found that only a few minor changes were required to get them ready for quantity production. I over-optimistically started advertising in the Canadian magazine Electronics Today International (ETI) -- see page 61 (p.77 in .pdf) in the February 1984 issue -- and in the July 1984 issue of '68' Micro Journal. But there was still lots of work to do to finish writing and testing the ST-MON monitor code, the FLEX Conversion Package code, and writing four user manuals, and this took longer than I expected. In June 1984 I finally shipped the first production bare board sets to a few customers, but with minimal documentation. The manuals (CPU, FDC, ST-MON 1.01, FLEX Conversion) weren't ready until the beginning of September 1984.

Microware's OS-9/6809 Level I operating system became available for the CoCo near the end of 1983 for only U$69.95 -- and that price even included an editor, assembler, and debugger! This was a real bargain compared to what OS-9 cost for other systems. I bought a copy and hand disassembled various modules such as Boot, Init, SysGo, Clock, and much of the kernel, to see how they worked, and what modifications I would need to make to them to work on the ST-2900.

I decided to license SDisk from D.P. Johnson for the floppy disk drivers and format program. This reduced the time required to write the drivers, provided support for several OS-9 disk formats, and provided features to support other software that could read and write other floppy disk formats, such as MS-DOS. I also hoped the SDisk drivers would be a good selling feature.

Like I had already done for FLEX, I decided to create a conversion program to make the process of adapting the CoCo version of OS-9 to the ST-2900 easy for the user. Since ST-MON 1.01 could only boot from FLEX format disks, the first version of the OS-9 Conversion Package (December 1984) was actually a bootable FLEX program.

An improved version of ST-MON (2.04) was created that could directly boot OS-9. This allowed me to create a second version of the OS-9 Conversion Package (August 1985) that was on a bootable OS-9 disk, and could create bootable OS-9 system disks configured for the ST-2900, which was much more convenient.

Along the way I received lots of encouragement from fellow members of the WCCS computer club that I belonged to, as well as from members of the Motorola User Group (MUG) in the Seattle WA area. They also provided technical help, corrections and suggestions for improvements to the software and user manuals, and purchased some board sets from me. (If anyone has any information on Will Wilgus, aka Lee Wilgus, who lived in Seattle in the 1980's, and was a great help to me for both the ST-2900 and DMC projects, please let me know, as I have lost contact with him.)

An ST-2900 system became my main development system, replacing the SwTPc system I had been using for the previous 6 or 7 years. Once I became familiar with OS-9, I started using it more than FLEX, as OS-9 was a far superior operating system.

Using a system with only floppy disk drives was becoming a pain, so I designed and built a prototype of a 512KB RAM-Disk board, and wrote FLEX and OS-9 drivers for it. (Fortunately I had included the /EXXX signal in the ST-2900 CPU board design to accomodate it.) Jeff Peters did an excellent job of creating the hand-taped artwork for the PCB layout. Adding a RAM-512 board to my main ST-2900 development system made a huge improvement to my productivity and enjoyment. Unfortunately I sold very few RAM-512 boards, so it was a financial flop.

Other accessories were also designed -- serial port expander, parallel printer driver, SASI adapter, SCSI adapter, and one or two RTC (real-time clock) add-ons -- but none of them were put into production. In hindsight I wish I had put more effort into fitting an RTC chip (with battery backup) into the ST-2900 design. (37 years later I designed two new RTC add-ons for the ST-2900, one using the DS2417 chip, and the other using the DS3234.)

Sales of the ST-2900 boards were slow, but came from all over the world -- I eventually had customers in at least 14 countries on all continents (except Antarctica). Although most were sold as bare boards (plus ST-MON in EPROM, and the FLEX and/or OS-9 Conversion Package on disk), I did eventually also sell several partially assembled as well as fully assembled and tested board sets. One repeat customer bought assembled and tested board sets that omitted the floppy disk controller, and had other modifications, that he used in some kind of telephone system. He typically bought 5 or 10 sets at a time, and the income from this helped keep me in business.

I did preliminary work on follow-on products, such as a 3 MHz HD63C09 system with MMU / DAT (refer to the explanation in the MP-09 assembly instruction manual) capable of running OS-9 Level II, that I tried to fit onto 3 or more tiny (business card size) boards! But none of these designs were ever completed or put into production.

In August 1986, as ST-2900 sales were only trickling in, I started developing a non-related product, the DMC "No Halt" Dual Mode Floppy Disk Controller for the Radio Shack Color Computer.

How many ST-2900 boards were sold? Somewhat over 200 of each of the CPU and FDC boards, and maybe a dozen RAM-512 boards, between 1984 and 1992. Not enough to make a proper living, even though I worked at it full-time for a few years, with only a little bit of other income on the side. I was surprised to sell a few more bare board sets years later in 2007, then in 2017 sold all my remaining inventory of ST-2900 boards and components to a customer in the UK.

Looking back, I wish I would have developed a second-generation ST-2900, with a 2 MHz 6809, 64KB of static instead of dynamic RAM, 4 serial ports, a buffered parallel printer port, a SASI or SCSI hard disk drive interface, and a battery-backed RTC. This would have better competed with Peripheral Technologies' PT69-5. Having 256KB or more RAM and a MMU/DAT to support OS-9 Level II would have been nice, but the affordable (U$79.95) CoCo 3 version of OS-9 Level II wasn't available until late 1986, and would have been more difficult to port to my system than Level I was.

In case you're wondering, the "ST-2900" name is composed of "ST" for Sardis Technologies, "2" for a 2-board system, "9" for 6809, and "00" to allow 01..99 for names of related products.

If you read this entire story, thank you for your interest!


"FLEX" was a trademark of Technical System Consultants (TSC).
"OS-9" is a registered trademark of Microware LP.
"Radio Shack" and "Color Computer" are trademarks of Tandy Corp.
"MS-DOS" is a trademark of Microsoft Corp.

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Last revised 2020-Jun-07 14:50 PDT.
Copyright 2020 by David C. Wiens.

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