Allow for multiple I2C interfaces

[soundanalogous]

The current Firmata implementation (and protocol) only allows for a single I2C interface per board. For Firmata 3.0, update the protocol and implementation to allow for multiple I2C interfaces (SCA0 / SCL0, SCA1 / SCL1, etc).

[rwaldron]

Excellent!

[PizzaProgram]

In this case please consider to make it possible to forcibly give unique IDs to each I2C port!
Because linux OS is adding numbers to each one at "initialisation order".
So it can happen, that USB0 gets a bit later initialized, because the board is still booting, or other I2C ports gets activated on a Pi4 >> and the ID=0-3 becomes suddenly 1-4 or 2 boards get's mixed and becomes 4-7, etc...

So it would be safer to order FIXED IDs to them when flashing the board. ID=111-114, Board2: 121-124 ...

I2C_example.ino should start with lines like this:
#define FIRMATA_I2C_ID0 111; // Sets first I2C ports ID to override OS auto-sequence-numbering,
#define FIRMATA_I2C_ID1 112; // which can differ each time the device boots up.
...

Or will be these I2C ports completely invisible for the OS and only available through "direct Firmata commands"?

[soundanalogous]

Are you asking about I2C or about USB specifically? The solution to adding support for multiple I2C ports is a non-backwards compatible update to the Firmata I2C interface.

[PizzaProgram]

For Firmata 3.0, update the protocol and implementation to allow for multiple I2C interfaces (SCA0 / SCL0, SCA1 / SCL1, etc).

I'm asking (here, at this topic) only about I2C.
Maybe I'm misunderstanding something, but your plan is to add multiple I2C support in Firmata 3.0?
If yes, you should consider what I wrote.

(Except, if those ports won't be reachable from outside the board firmata is installed onto.)