Sardis Technologies ST-2900 RAM-512 Board

Sardis Technologies RAM-512 board

The ST-2900 RAM-512 board is an expansion board that plugs into the ST-2900 CPU board to provide 512KB of extra RAM, typically used for a RAM-Disk or a printer buffer.

RAM-512 board features:

(The new NVRAM/RTC board has 4x the capacity, yet is smaller, costs less to build, draws less power, and is non-volatile.)

Once I had built and installed a RAM-512 board in my main ST-2900 computer system, I was hooked! It was so much quieter, faster, fewer floppy disk swaps, less wear and tear on the floppy disks and drives, etc. It was bliss! Fortunately I lived in an area where power outages were rare, so I left the system powered up 24x7 and usually didn't lose any data for half a year or more. (Of course I backed up the RAM-Disk contents frequently, just in case.)

One of my industrial customers paid me to write custom software to allow using an ST-2900 CPU and RAM-512 board set as a large printer buffer.

The RAM-512 board is slightly larger than the CPU or FDC boards, to allow for two 60-pin connectors. At the time I designed the board I couldn't find an appropriate stack-thru connector (like the PC/104 bus used 2 years later), or at least not one that was affordable.

The RAM-512 board was announced on page 48 of the September 1985 issue of '68' Micro Journal, and reviewed on page 16 of the June 1986 issue.

Thinking recently about the design of the RAM-512 board, I'm not sure why I gave it a large 4KB window, instead of only a 256-byte (1-sector) window, or an even smaller 2-byte window with an auto-incrementing counter (maybe located at $FF60). The large window makes it more difficult to use that address range for code, as any code located there temporarily disappears from view whenever a different RAM-512 data page is enabled.

One of the features that was included in the FLEX driver for the RAM-512 board was a patch to the FLEX LOADCODE ($CD30) routine to greatly speed up loading programs, especially from RAM-Disks. I have now extracted that code into a separate standalone program, called FASTLOAD.CMD, that can install this patch on any 6809 FLEX system, not just the ST-2900 with a RAM-512 board. The new code (185 bytes) is installed below MEMEND. Note that this patch can only reduce the time to load a program from a floppy disk if that disk has been formatted with a smaller than usual interleaving factor -- a factor as small as 2 (every-other-sector) should work. Refer to the .zip file below.

(Click on a picture for a larger version.)

icon to link to photo of top of bare RAM-512 board icon to link to photo of bottom of bare RAM-512 board
Pictures of bare board. (943-981KB .jpg)
icon to link to photo of top of assembled RAM-512 board icon to link to photo of angled view of top of assembled RAM-512 board icon to link to photo of angled view of top of assembled RAM-512 board icon to link to photo of bottom of assembled RAM-512 board
Pictures of assembled board. (597-895KB .jpg)
icon to link to scan of PCB artwork for top side of RAM-512 board icon to link to scan of PCB artwork for bottom side of RAM-512 board
PCB artwork (first version). (176-225KB .png)

icon to link to RAM-512 User Manual ST-2900 RAM-512 Hardware and Software User's Manual.
(4.4MB .pdf scanned image)
icon to link to additional notes for RAM-512 board -- not available yet Other notes for the ST-2900 RAM-512 board. Includes addendum/errata to the manual, etc.
(not available yet)
(??KB .txt)
icon to link to .zip file with DSK images of FLEX and OS-9 drivers for RAM-512 board Two DSK images (35T/SS/SD FLEX format, 35T/SS/DD CoCo OS-9 format) of source code and binaries of FLEX and OS-9 drivers for ST-2900 RAM-512 board. Also includes bootable version of MEMTEST.
(25KB .zip)
icon to link to .zip file with source code and executable of FASTLOAD.CMD Source code and executable for FASTLOAD.CMD, plus large test file.
(5KB .zip)

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

Last revised 2024-May-09 10:48 PDT.
Copyright 2018- David C. Wiens.

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