On Sun, 07 Jul 2013 10:59:13 -0300, you wrote:

>I don't think it is easy (or even possible) to implement CSMA/CD in an
>RS-485 bus.
>I tried it once without success, each node can only see its only
>transmission.
>It seems that because the RS-485 transceivers drive both levels
>actively, and together with the bus resistance makes the distorted
>levels undetectable.

One solution if you need to get a lot of data back from multiple nodes is t=
o allocate timeslots
based on device ID, relative to a single broadcast poll request.=20

When using USB-to-RS485, USB turnround delays can add a lot of delay to ind=
ividual poll/reply
systems, but this is only an issue at higher throughput (100+kbits/sec and =
many dozens of nodes).

Not relevant to this application, but can be a useful technique, e.g.  for =
things like this, which
had 150 nodes (across four RS485 buses) polling 250kbit/sec data at about 6=
0Hz:
http://marshmallowlaserfeast.com/82985/1310941/home/forest


--=20
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