For the back off time I suggest you use each device's ID number.  Then place
the faster re-trying devices in the most important parts of your network.
Simple, effective and intutitive for the installer, lower numbers mean
better comms, higher numbers mean lower priority.

As to the echo verification method - its is used in several places.  One
that comes to mind is Texas Instruments graphical calculators - they use a
simple 3 line, 2 way, echo verification comms setup; see their site for
details.
You could expand this with the IEEE-488 Talker/Listener scheme where an
extra line is pulled low by *ANY* device to initialize comms with the Master
(who then sets up a private conversation by telling all other Slaves to
UNListen until the bus is released).

Bye.

-----Original Message-----
From: Brandon Fosdick [mailto:bfoz@GLUE.UMD.EDU]
Sent: Tuesday, 17 July 2001 11:13
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]:Thoughts on RS485 collision detection


For my project that uses an RS485 network I was planning to use a token
ring system so I don't have to deal with collision detection. But I got
to thinking about it and now I'm wondering if it might be worth doing.

The solutions I've seen posted so far (and on piclist.com) all seem to
be of the bitbang variety. Since I'm using 16F877's for this project it
seems like a waste to bitbang stuff when I have a nice USART to use. Has
anybody done this yet? I'm thinking that if I leave the receiver enabled
(im using a max487) the USART should receive a byte for every byte that
it sends. If the two bytes are different a collision has occured. First
off, that sounds too simple, like something that somebody has already
thought of and rejected. Second, whats a good way of handling back-off
and retransmission?

If all the devices are set for the same back off time, or time
increment, they'll just collide again. I guess I could implement a
random number generator, but I'd rather not. Is there a good way to
handle this?

How should retransmission be handled? Obviously the sending device needs
to retransmit the lost data, but how does the receiving device know that
a collision has occured? If it doesn't know there's been a collision it
might think the corrupted data is valid.

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