Jan-Erik Soderholm wrote:

>> (Note that due to the bus arbitration, you may not have two devices that
>> may try to send the same message id.)
> 
> I read it such as you aren't even allowed to have multiple devices
> potentionally sending using the same ID !? 

I'm not sure I understand you here (or you me :) as I'm considering our two
phrases identical. 

The arbitration mechanism works on the message id (plus the RTR bit). CAN
arbitration doesn't work if two devices potentially may try to send the
same arbitration data, so that's a no-no.

There's an issue with that with the RTR frame mechanism. (AFAIK several of
the higher level protocols don't use them because of this.) The reason is
that if you have multiple consumers of a data point/message id, these may
want to be able to request that data independently from each other. Using a
simple, straightforward RTR mechanism, that's not possible, because it
could create a situation where different devices send the same message id
with the RTR bit set. I worked around that by encoding the sender into the
message id (and using a bit of the message id for marking requests). That
guarantees that all requests are unique.

Gerhard
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