2010/7/5 Ruben J=F6nsson <ruben@pp.sbbs.se>:
> As others have said you can handle the frame decoding in the ISR itself a=
nd
> only call the processing_thread when you have a complete frame in order to
> increase efficiency.
>
> Another thing I havn't seen mentioned here yet is the potential problem w=
ith
> collitions which have to be addressed since you are planning on making a
> multimaster protocol. A collition will happen if two masters tries to ini=
tiate
> a message at roughly the same time. If you are planning to use standard R=
S485
> tranciever chips the masters themselves may not even notice the collition=
 even
> if you read back the transmitted data since they actively drive the line =
to
> both polarities.
>
> This can be solved with a bus that is only actively driven to one polarit=
y and
> passively pulled to the other. This is how a CAN bus works.
>
> If you don't want a bus that is only actively driven to one polarity (it =
will
> be more succeptible to noise in the passive state) there are other method=
s to
> try to minimize the chance of a collition between masters to a low enough
> probability that it can be acceptable. One such way is to wait a random t=
ime
> after the bus has become quiet until the master starts to transmitt the n=
ext
> frame. In the unlikely event that two masters still will transmitt at the=
 same
> time and not notice each other, the message will be messed up anyway and =
the
> checksum/crc will catch the faulty message. Of course, you will need some=
 sort
> of ACK mechanism so the master knows that the message has not been receiv=
ed for
> this to work. Instead of using a random backoff time, you could use a time
> based on the unique identifier address (which most likely exists anyway) =
for
> the master. This could give masters with a lower address a higher priorit=
y.

Hi Ruben,

I had another thread a while ago specifically addressing issues with
multi-master protocols and collision detection/avoidance over RS485. I
just have to go find it now and re-read it.

I love these discussions. So much useful information.

-- =

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