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-----
> From: Kenneth Lumia [mailto:klumia@ADELPHIA.NET]
> Sent: Monday, December 17, 2001 4:21 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: How can I power a small RS485 network?
>
>
> Hello,
>
> 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.
>
> Ken
>
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