>I am thinking of using RS-485 for hooking together some home automation
>bits which all can Tx & Rx. The Rx bit is easy but what if two or more
>devices try to Tx at the same time ? Collission detection is obviously
>required but am unsure of how to implement it on serial line. There will
>be no "master" in the system so a polling arangement is out. I did have
>the thought of when a collission is detected the unit could wait for the
>line to clear then try again. As each unit will have a unique address, I
>thought that I could use this as a variable for re-try timing to stop the
>units clashing more than once each time. Does this make any sense ?
>The serial data would be made up of 10-byte packets with header,crc & tail.
>I think there would be plenty of uses for such a multi-drop framework.
>Please tell me if I'm heading up that famous sh** creek !  :-)

Your idea of data transmission is the same with that used in CSMA/CD systems
(Carrier Sense Multyple Access/ Colosion Detection ) ( the Ethernet standard
is the best example ).

Speaking about RS485, I dont't know how you detect the colosion ( multyple
active transmitters )> You can try for example to receive the transmitted
data at the transmitting station and check it to see if it was transmitted
correctly .

Two years ago I built an RS485 communication system, but with a master
device ( a PC ) which polls slave devices ( Philips 80c552 based ) for
an interactive TV application.

                                        Dorin
---------------------------------
Dorin Dogaroiu
Research Institute for Computers
Bucarest, Romania
Email : dogaroiud@pcnet.pcnet.ro
        ddorin@kappa.ro
---------------------------------


>rEGARDS.  Sorry... cOuLdN't hElP mYsElF !! (Yeah, I've been told already !)
>
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