Sardis Technologies ST-2900 CPU Board

(This page is still under construction.)

The ST-2900 CPU board is a single-board computer designed around the Motorola 6809E 8-bit microprocessor.

While the ST-2900 CPU board can run by itself, it also has an expansion connector that allows plugging in other boards such as the ST-2900 FDC and RAM-512 boards, or custom I/O boards.

CPU board features:

• Motorola 6809E 8-bit microprocessor @ 1 MHz
• Signetics 2681 DUART:
  • two serial ports (both receivers quadruply buffered, hardware handshaking)
  • four baud rate generators (up to 115200 bps)
  • one 16-bit counter/timer
  • 7 input and 8 output lines
• 64KB (or 16KB) DRAM using eight 5V-only 64Kx1 (or 16Kx1) chips
• Motorola MC6883/74LS783/SAM: DRAM multiplexing and refresh, and device decoding
• 24/28-pin EPROM socket: supports 2KB/4KB/8KB devices (up to 64KB with modifications)
• 60-pin connector plugs into FDC or RAM-512 boards
• only 3.9" x 6.3" (100mm x 160mm), 2-layer board with soldermask and silkscreen

Almost 35 years after designing the 2681 DUART into the CPU board I discovered that although the original data sheet indicated the baud rate generator's fastest rate is 38400 bps, it has a "test" mode that allows faster rates up to 115200 bps! Later versions of the data sheet document this mode.

Pictures of bare board (453-512KB .jpg), assembled board (453-552KB .jpg), and red/blue PCB artwork (473KB .jpg).

	ST-2900 CPU Assembly Instructions and User's Manual. (8.0MB .pdf scanned image)
	Other notes for the ST-2900 CPU board. Includes addendum/errata to the manual, modifications for 16KB/32KB/64KB EPROMs, etc. (not available yet) (??KB .pdf scanned image)
	The ST-2900 ST-MON 2.04 monitor program initializes the hardware, performs self-diagnostics, provides various I/O routines, and has commands to view/search/move/fill/modify/test memory, and to boot the FLEX or OS-9 operating system from floppy disk.
	The Serial Port Expander (SPE-1) lets one serial port on the CPU board connect to 3 serial devices (one at a time).
	RTC-2417 board adds a real-time clock (RTC) with battery backup to the CPU board, using two spare pins from the 2681/DUART.