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Another comment: other bus types

So you had the "wrong" idea about the serial being a bus?

In fact, it is wrong an idea when we are talking about the RS232 or the logic-level serial on the Arduino.

But there are serial bus systems. Research the RS485 standard for example.
This can be implemented using the AVRs serial port and a simple diver chip like the SN75176 or its low power cousins from Maxim.
the tricky part is the bus arbitration: no two senders may send at the same time. Usually this involves a master-slave scheme of some kind.

I am using RS485 type buses for all the long-haul communications where TWI is really at hte limit.
When using 5bit datawords (5n2 is the smallest the AVR serial supports, iirc), then the clock difference between the devices on the bus may even be 3% or so - usually communication is stable even with devices running off the RC oscillator (without a crystal).

For linking a bunch of Arduinos this is a very good choice for hte long distance: cheap drivers, good isolation (worst case: a driver chip burns out) and loooong distance is possible (one through your garden and back).
Drawback is bandwidth: on a long line only like 4800 or 2400 bit/s are really usable.



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