On Wednesday 25 May 2005 12:32 pm, Olin Lathrop scribbled:
> ...  What happens when a node has something to say, but it can't
> just squawk it out?  Do you have a master that queries each node in turn? 
> How many nodes does it query?  What happens when a queried node goes down? 
> Do you wait for some timeout?  How do you detect new nodes being added? 
> Will you leave enough room in your home grown protocol to accomodate future
> expansion?


So far, my thoughts for most of this are as follows... 
- Master will query each node in sequence asking "do you want to send 
anything"?
- Master will wait for a specified time for a response.  No response in 
specified time means an error occurred.  I've been toying with the idea of 
re-trying once or twice, but haven't determined that yet.  Any error is 
indicated at the master station (specifics TBD).
- When the master gets a response, it will be addressed on the bus to the 
master, but the data will indicate if it is intended for another station, and 
the master will pass it along.
- I'm planning to implement an acknowledgement to the slave that message was 
received.
- Master will send out message on the bus, which the appropriate slave will 
respond to.
- Master will expect an acknowledgement within a certain time, else error.
- Master will continue with next node.
- I might need a broadcast message, but will not expect an acknowledgement for 
that.  If I do, I'll have the master send it to all stations 
individually/sequentially.  Latency is not an issue here.

At this point, there will be about 10 stations, but I was planning to allow 
for 16.  However, for simplicity, and since traffic will be relatively low, 
I'm thinking I'll just send a full byte as the station id/address, and 
another with the command, and perhaps a third with some checksum, etc.  That 
last part is yet TBD.

The messaging will be very specific to each station, so new nodes will require 
a firmware change at the master, since it will need to be setup based on the 
type of node.  I don't plan to put anything automatic in for this.

Cheers,
-Neil.


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