Uses a plug-in Adafruit Isty Bitsy as the processor. This in turn carries a ATSAMD51 Cortex M4 processor running at 120 MHz and is supported by the Arduino tool chain.
Small form factor, nominally 2.4 x 2.4 inches (60x60mm).
Daughter boards can be stacked above or below the core processor.
20-way backplane using stackable headers.
Backplane has provision for I2C, SPI, 2x UART, analogue, and of course digital.
"Self-enclosing" physical design whereby small PCBs can be soldered to the edges of the main PCB at right angles to form sides and make a box . Add a "lid" to enclose the electronics and optionally provide a user interface.
These solderable sides also have access to 18 of the backplane signals and can therefore hold electronics. The current design has a signal breakout side for debugging, a power supply side, and an RS-485 side.
Provision for a small expansion PCB on the main board, useful for a real-time clock or small OLED display.
This is the new design. After a few years of inaction I've come up with a modified design that is much simpler and yet more powerful in most ways.
The original design only had a single solder-on side, this new design has four, all of which can house electronics and/or connectors for IO interfaces.
Another big change is the processor. Despite the fact that I love their chips and tool chain, after total lack of support from NXP I've gone with an off-the-shelf design in the form of a tiny carrier board with a ATSAMD51 Cortex M4. This is an Adafruit product that has good support and is Arduino ready. Therefore I will spend almost no time debugging the processor electronics and fighting with the development toolchain only to wind up with something that nobody will want to use because it's not "Arduino inside".
Of course, as the processor is on a carrier board, and the backplane is very generic, a new processor version can be designed at any time and just plugged in.
This project has languished for a long time but I think I will resurrect it.
The format will change but still be similar in many ways, with stackable boards and solder-on front panels. The main difference is that the new design uses an LPC1549 ARM processor and has built in RS485 networking using a protocol I am designing called R3N (Robust Redundant Ring Network).
Why bother when you can run Linux and WiFi or Ethernet on a board 1" square these days? I dunno, I just like doing this stuff and I can see uses for it.