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(Last updated: Tuesday, April 03,2018)

Sardis ST2900

When I graduated from college as an EET I had the typical run of 'you have no experience' that many run into today. And while graduates have better resources than I had I did manage to do one thing right. I built an EPROM Burner and wrote the code in 6502 assembler for my Atari 800XL. That helped me to get a job with Micro-Comm in East Brunswick NJ. This Micro-Comm made communications equipment for the newspager industry. Since this was the early days of the microcomputer I got to learn all sorts of interesting things; protocols, hardware and software. One of the interesting computers I got to use was the Gimix Ghost running OS-9 Level II (Multi-user, multi-tasking). This was a 2MHz 6809, 8 bit, based computer, it had 2 Meg of RAM (at a time when 64k cost more than $100), a 20M hard disk, 8 serial ports and a parallel port. This was our development machine. We ran 7 people off this machine for development and communications. I really wanted one but it was expensive. One interesting thing about the machine was that it's IO boards had their own CPUs. Even today it's a hard machine to find. But along the way I ran into a strange little computer call the ST-2900 by Sardis Technology (David C. Wiens). It could run the STAR-DOS, Flex or OS-9 operating systems. The OS-9 was the version from Radio Shack, used on the Coco computers. This version was less expensive than buying OS-9 from Microware but was the same OS-9. With just a little tweeking of a few of the modules you could gen (cobbler?) an OS-9 disk that would turn the ST-2900 into a smaller version of the development system we ran on the Ghost (no CPUs on the IO devices and less memory).

So I purchased a ST-2900 and got to work. I had to find a case and a power supply with enough power for the computer and the floppy drives. Then I needed to build the connections to the serial ports and the parallel ports. Then wire the floppy drive cable and power the drives and the computer and hook it up to the serial port. I ended up creating a hodge-podge of 40T - 5 1/4 inch, 80T - 5 1/4 inch and 80T - 3 1/2 inch drives (6 drives total) on the cable. 3 1/2 inch drive were new and we had a few spare. I had to have the 40T and 80T, 5 1/4 inch drives as the rest of the systems we worked with supported these formats. And yes, it's possible to support more than 4 drives on a drive cable. If I recall correctly, you could have 8 drives (multiplexed).

Once the system was setup, it was time to start up the computer. If all went well you were greeted with a simple 'CW? ' and the cursor. That meant you now had access to ST-MON and could various commands to boot the computer. I recall writing a simple assembly language program to start and stop each floppy drive's motor. This allowed me to check that I had built the cable correctly.

The next part is a little fuzzy. I recall booting the OS-9 conversion disk and then building the OS-9 boot disks. I recall this being quite difficult and I must have spent more than a week failing to boot the system into OS-9. I do recall that at some point I was able to get the system to boot. Since I still have the disks I know I must have done something right. ;-).

While all of this was a nice walk down memory lane it also serves as a reminder of what I'll soon run into. I'm working on getting these machines up and running. Which means I need to find some drives, power, a case and make this bootable. At the moment getting a new triple output (+5, +12, -12v DC) power supply isn't cheap. At the time they were as common dirt and about as cheap. Yes, I am thinking of using a simple PC power supply.


  • CPU - Motorola 68B09, 8 Bit, 2MHz.
  • RAM - 64K 150ns DRAM
  • USART - 2 serial port via the 2681
  • PIA - 6522
  • Floppy - FDC1793
  • SAM - 6883 or 74LS783
  • RAM Disk - 512K

To get the computer running a lot of prep work was needed. I had to get the correct power supply of, at least: +5v@3A, +12v@0.5A, -12v@0.3A. Then I need a case, the RS232 connectors (I'll use DB9) and at least two 5 1/4" floppy drives. Though I am hoping to get a floppy drive emulator. Then I'd get to work gen'ing a new OS9 boot disk for the ST-2900.

One thing I never got around to when I first put this machine together was adding on the SCSI interface. I had also purchased the SCSI interface for the ST-2900 which consists of a couple of 74LSxx chips and an NCR5380. I'll really need to sit down, add that on to the computer then write the appropriate device driver in 6809 assembly language.


Friday, April 13,2018 - I found the third Sardis ST-2900 board assembled in a clone PC case with a PS/2 130W power supply and a 40 track 5 1/4 inch floppy drive. I forgot I had built this back in 1984. The case and power supply are cetainly TCF finds.

    Okay, one boards returns M2CW? and won't echo back anything (not a good sign -
    RAM). I'll need to look into the RAM and see if it's a corrosion problem or bad RAM/. The
    second board is better:


        ST-MON 2.04 (c) 1984 BY DAVID C. WIENS
        =E FE7F FE96
         FE70  4D4F4E20 322E3034 20286329 20313938  MON 2.04 (c) 198
         FE80  34204259 20444156 49442043 2E205749  4 BY DAVID C. WI
         FE90  454E5304 3D040D0A 00000000 04FFFF00  ENS.=...........
At 9600 7N1 (the IPx pins are set to 9600 8N1, hmph). I haven't tried board 3 yet. I've worked on the SCSI board and figure out most of it. Now I need to figure out the input lines (some are not obvious). So that down to a few pins to figure out. I know it's a combination of the 6809 control lines such as BA, BS, E, MREADY, Q and R/W. Now if I can only recall what they all did. I now need to clean up the disk drive and I hope I'll have something bootable soon.

      Vss  I1          40I Halt
      NMI  I2          39I XTAL
      IRQ  I3          38I EXTAL
     FIRQ  I4          37I RESET
       BS  I5          36I MRDY
       BA  I6          35I Q
      Vcc  I7          34I E
       A0  I8          33I DMA/BREQ
       A1  I9   6809   32I R/W
       A2  I10         31I D0
       A3  I11         30I D1
       A4  I12         29I D2
       A5  I13         28I D3
       A6  I14         27I D4
       A7  I15         26I D5
       A8  I16         25I D6
       A9  I17         24I D7
      A10  I18         23I A15
      A11  I19         22I A14
      A12  I20         21I A13

SCSI Interface

I have a small 53C80 (SCSI) interface chip board for the ST-2900. Unfortunately I don't have a schematic or description of how to interface it. I've managed to get most of the pinout figured out but not all of it. Add to the fact that I haven't built an interface for a 6809 since 1984 and you have a mystery I need to work out.


      Pin | Description
      1    D0
      2    D1
      3    D2
      4    D3
      5    D4
      6    D5
      7    D6
      8    D7
      9    A2
     10    A1
     11    A0
     12    Vcc
     13    Gnd
     14    U1 (53C80 p??)
     15    U3 (74LS00 P13 - A)
     16    U3 (74LS00 P1  - B)
     17    U2 (74LS139 P14 - 2A)
     18    IOR (Need to derive this from R/W)
     19    IRQ
     20    U2 (74LS139 P4 - 1Yb)
     21    u2 (74LS139 P4 - 1B)
     22    u2 (74LS139 P4 - 1A)
     A | B |EOP| Result 
     0 | 0 | 0 | 0 
     0 | 0 | 1 | 1 
     0 | 1 | 0 | 0 
     0 | 1 | 1 | 0 
     1 | 0 | 0 | 0 
     1 | 0 | 1 | 1 
     1 | 1 | 0 | 0 
     1 | 1 | 1 | 1 

53C80 Chip

       D0  I1          40I D1
      DB7  I2          39I D2
      DB6  I3          38I D3
      DB5  I4          37I D4
      DB4  I5          36I D5
      DB3  I6          35I D6
      DB2  I7          34I D7
      DB1  I8          33I A2 (in)
      DB0  I9   6809   32I A1 (in)
      DBP  I10         31I Vcc
      Gnd  I11         30I A0 (in)
      SEL  I12         29I IOW (in)
      BSY  I13         28I RESET (in)
      ACK  I14         27I EOP (in)
      ATN  I15         26I DACK (in)
      RST  I16         25I READY (out)
      I/O  I17         24I IOR (in)
      C/D  I18         23I IRQ (out)
      MSG  I19         22I DRQ (out)
      REQ  I20         21I CS (in)

     A2 | A1 | A0 | R/W | Result 
      0 |  0 |  0 |  0  | Output Data Register (write)
      0 |  0 |  0 |  1  | 
      0 |  0 |  1 |  0  | 
      0 |  0 |  1 |  1  | 
      0 |  1 |  0 |  0  | 
      0 |  1 |  0 |  1  | 
      0 |  1 |  1 |  0  | 
      0 |  1 |  1 |  1  | 
      1 |  0 |  0 |  0  | 
      1 |  0 |  0 |  1  | 
      1 |  0 |  1 |  0  | 
      1 |  0 |  1 |  1  | 
      1 |  1 |  0 |  0  | 
      1 |  1 |  0 |  1  | 
      1 |  1 |  1 |  0  | 
      1 |  1 |  1 |  1  |