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'[EE]: How can I power a small RS485 network?'
2001\12\17@163534 by Kenneth Lumia

picon face

I need the lists knowledge of RS485 to answer a hopefully simple question -
can I use extra pairs in a Cat5 cable to power several RS485 nodes? More
details below (long).

I'm creating a small RS485 network in my home to monitor temperatures,
humidity, water leaks and a few security sensors (using PICs of course).
The nodes are relatively simple, however I am somewhat at a loss on
how to power them.  The simplest solution is of course a wall wart on
each node, however I want the nodes to look "clean", that is no wires
hanging outside the walls, very similar to an air conditioning thermostat
housing.  I plan to run a daisy-chain of Cat5 cable to each node.
Now the question.  Has anyone ever run power to RS485 nodes on
unused pairs of Cat5 cable?  Is this a no-no? The power requirements are
relatively low, < 100mA each node during transmission, maybe
10 - 20 mA at other times, x 6 to 8 nodes.

I'm thinking a small supply, either AC or preferably DC (to reduce parts
count on each node) at the start of the cable is a possible solution, i.e.,
a DC supply of sufficient voltage to cover power losses, L di/dt, etc.,
with a LDO regulator at each node.  Is this poor design Vcm wise, etc?

I've seen many RS485 designs use 100 ohm resistors in series with the ground
connection to limit common mode current.  Are they really needed on a
"small" installation (300 feet, internal to one building).  If needed, they
sort of mess up the idea of using DC.

Any thoughts, suggestions?

Thanks in advance.


-- hint: To leave the PICList

2001\12\17@165944 by Douglas Butler

Basically yes.  Look at the DC resistance losses of your wire and decide
if voltage losses will be a problem.  Note that RS485 built according to
the spec is quite tolerant of common mode voltages.  I don't expect you
will have a problem.

But if voltage losses are going to be a concern you can power the remote
nodes with a floating current source.  Just put a zener at the load to
set the operating voltage.  Separate current sources for each node might
be a hassle, but if you have a long wire to a cluster of nodes they can
share the same current source and zener.  We do that commonly in
oceanographic instruments at the end of long (kilometers) cables.

Sherpa Doug

> {Original Message removed}

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