At 09:12 PM 16/07/01 -0400, you wrote:
>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?

In practice, the local receiver will see an echo of what you sent even if
there was a collision. This happens because of the voltage drop in the
network cabling and due to the receiver only needing a small differential
voltage to make it work. It won't see the fault which is farther away.

>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?

Do the nodes each have an address? If so, base the retry on the address.

>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.

Frame the data in packets with unique start & end characters.
Is there a master in the system that can poll the slave nodes?
Regards...

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