solarwind wrote:
>> Finally, why do you feel you need a token-based network?
>
> Got any other ideas?

I work with automotive protocols a lot, and the J1850 approach to avoiding 
collisions sounds like it would work in your case.

There are two states: dominant and recessive. Let's say you have weak 
pull-ups on your bus, and each node can only drive the bus down. So positive 
level is your recessive state, and "ground" potential on the bus is your 
dominant state.

Let's say now that two nodes start transmitting at the same time. While they 
are transmitting, they are also monitoring the state of the bus. If a node 
transmits a recessive bit, but reads it back as a dominant, it knows it lost 
arbitration, and has to wait for idle bus again. The second node continues 
transmitting as if nothing had happened.

Same thing happens behind the scenes, on CAN.

I don't see any advantage to a token-based network, unless your events are 
very time-critical, and your bandwidth is very limited.

Vitaliy 

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