We've been asked to document the pin allocations that we use for the I2C connectors on our boards.
Here they are:
The Int line is used to signal an interrupt from those boards that can generate one, including the MCP23017 Port Expander Board.
The 3v3 line is powered by the Interface Boards' voltage regulator, and can provide a much higher current than the Pi's 3.3 volt supply.
Great. How about doing the same for the SPI connectors
I will. I just need to get it checked before I publish it.
What voltage level do the SDA and SCL lines run at?
I2C runs at 3.3 Volts. We level shift the serial lines to 5v, but not the I2C lines.
We may make that a configurable option in a future revision. However, all the add-on boards (Port Expander and Analogue Boards) are already switchable between 3.3 volt and 5 volt operation.