It was painful getting the RPUpi shield to work. Mistakes with IOFF were the gift from hell. But the outcome feels extra useful, so now I want to have a way to connect a Raspberry Pi 3 to a RPUBus and have some SPI options. I have some doubts if this schematic is right yet so keep that in mind, but now let me brag...
SPI pinout for Adafruit Micro-SD breakout board (a SPI slave device). I want to log flow meter data with an RPUno. I think that could almost be an attractive product.
SPI pinout for connecting to a SPI master, it has IOFF (oh snap!) so the Raspberry Pi 3 can be powered off without screwing up the interface.
UART pinout for Adafruit FTDI-Frind, but really it is for connecting to a Raspberry Pi 3 (or ilk) and has the (dreaded) IOFF so the Pi can be powered off without locking up the RS-422 bus (did I say that was painful to get working on the RPUpi).
The UART pinout has an added pin to tell the host to halt.
I2C second port (e.g. SDA1 and SCL1) is on the ATmega328pb but not on the ATmega328p. I don't have a 328pb yet and Digikey is out of them. I think the toolchain in Ubuntu 17.10 is going to have it from what I can tell so when it is in stock it may finally be time to give the part a try. This may permit the Pi to talk out of band with the bus manager, and that sure does sound like a better way to set the bootload address and such stuff.
The layout has taken some time to finish, but it is done
Hardware files for some of the versions are at https://github.com/epccs/Eagle/tree/master/RPUadpt
Add RS-422 to a Controller node
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