Hi... I would hazard a guess that grounding your spare pair would be the 
best use.  When you say double-up the comms, I assume you mean using 2 
wires for each of the +/- of the balanced pair, and not having two 
wholly separate rs485 networks on the same cable.

Have you looked at the "unit load" of these 70 devices, and can the 
transmitter handle it?  And if the 70 devices can talk back then every 
one of them has to be able to drive 70 inputs...

Also, if the 70 devices aren't using an isolated input, you may run into 
problems due to noise on the balanced pair from the 24V (how much 
current?) running close by for long lengths.  I have no idea how to 
predict the induced voltage, but nonisolated rs485 transceivers usually 
have a fairly low common-mode voltage spec.  It might be a good idea to 
have your main controller be fully isolated, at least.

Anyway... what I'd do is have the comms be one pair (and of course make 
sure the pair are actually 2 wires that are twisted together in the 
cable!) and ground the spares.  Having a backup plan might be a good 
idea as well, it may be that trouble only comes up when nearing 
completion of the install and the 24V currents are high and lengths are 
long and non-ideal stubs/branches are in place, etc.  Using a CRC or 
somesuch integrity check in your data, if possible, and having a means 
to know what % corruption is happening would be real useful in such 
nonideal installations.

Good luck!
J



David Duffy (AVD) wrote:
> I am planning to deploy a network of (approx 70) remote devices via
> CAT5E cabling. It will carry 24Vdc as well as RS485 data at a low speed
> (2400 baud at a guess).
> 
> I've selected the low baud rate so that I can get away with a mixed
> topology (part daisy, part star) that the building layout imposes. The
> longest cable run from the central controller should be less than 300
> metres.
> 
> All of the terminations are via 'Phoenix style' 0.2" plug-in connectors.
> To lessen the voltage drop on the 24Vdc wires, I plan to double them up.
> 
> It got me thinking that since there is still a spare pair in the cable,
> should I double up on the comms as well? What are the implications of
> having two balanced pairs in parallel, bearing in mind the low data rate?
> David...
> 
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