Multi-Master RS-485 System

[robhol]

Has anyone built such a system (2-wire)? I have searched for a simple protocol to implement
in a PIC (876) "everywhere"but haven't found anything so far, in fact I haven't found any protocol at all...

For example I found this text and yes I agree with the text, but isn't there a standard already for this?

"Each node in a multi-master type RS-485 system can initiate its own
transmission creating the potential for data collisions. This type system
requires the designer to implement a more sophisticated method of error
detection, including methods such as line contention detection,
acknowledgement of transmissions and a system for resending corrupted data."

Any help is appreciated, a link to a good site or whatever!

/Rob

[kender]

My first thought was MODBUS, but MODBUS is a single-master bus.

If you are building a system from scratch and you need multi-master capability, I would recommend CAN. It's noise immunity and cable requirements are pretty much the same as RS-485. However, you will implement your system quicker with CAN.

[robhol]

Thanks for your answer!

I have seen MODBUS - it is ok, but master/slave.

CAN is of course ok, but I have most of the hardware already.

So I still appreciate any help to find a protocol, if there exist one?!

/Rob

[SherpaDoug]

You could build your own token-ring. Nodes just pass the token until someone needs to be master. Then they catch the token, do their master stuff, and re-launch the token. If a node does not see the token for a looong time it assumes the token was lost and launches a new one. If somehow two tokens exist they collide and are destroyed. Then a new token is generated. Make the token short enough to fit in a UART buffer so it can wait there if a node is very busy.

I an sure there are formal versions of this but I have never seen one.
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