Sardis Technologies ST-2900 NVRAM/RTC Board

(This page is still under construction.)

(Currently in design stage)

The ST-2900 NVRAM/RTC board is an expansion board that plugs into the ST-2900 FDC board to provide non-volatile RAM-Disk and Flash-Disk and real-time clock (RTC) functions.

NVRAM/RTC board features:

Comparison to old RAM-512 board:

The Maxim/Dallas DS3234 RTC is temperature compensated (with a TCXO), so typically loses or gains less than 5 seconds per month. (Another way to add a real-time clock to the ST-2900 will be with the RTC-2417 board.)

The drivers will allow dividing the SRAM and Flash portions of the NVRAM-Disk into multiple FLEX and (Nitr)OS-9 partitions.

A planned future revision to ST-MON will allow it to boot FLEX and (Nitr)OS-9 directly from the Flash-Disk, eliminating the need to have any floppy disk drives attached to the ST-2900.

Note -- to keep the driver software to a reasonable size and complexity, the Flash-Disk will normally be read-only. Writes to Flash will only be done with a special program that writes an entire partition at once, by copying an entire floppy disk, a RAM-Disk partition, or a virtual disk on a host PC via Drive-Wire.

An SPI-compatible (Serial Peripheral Interface) bus connects the SRAM, Flash, and RTC devices to the 6522/VIA on the FDC board. Although the 6522's shift register can't implement a full-duplex SPI host, all the slave devices on the NVRAM/RTC board only communicate in one direction at a time, using either the MOSI (SI/D/DIN/DQ0) or MISO (SO/Q/DOUT/DQ1) lines, but not both at the same time, so this works.

It is possible to assemble a subset of this board with only easy-to-solder thru-hole components. This limits you to a 1MB SRAM-Disk. The Flash and RTC chips and their supporting devices are all surface mount. All the surface mount devices are on the bottom side.


(Click on a picture for a larger version.)
Tentative board layout.


"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-Jun-25 15:39 PDT.
Copyright 2020 by David C. Wiens.

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