I was considering two pairs in parallel for the single comms link, as in
(for example) white/blue & white/green for D+ and blue & green for D-.

We're using 1/8th load RS485 transceivers so 70 units should pose no
problems.

The current draw from each unit is very low (20mA or so max), so I'm not
expecting a lot of drop in the 24V line.

Our other option of course is to triple the power wiring.

The data protocol is an existing one we've used many times before and
has a checksum. The consequence of an occasional bad packet would be
mildly annoying, but not critical.
David...

Jesse Lackey wrote:
> 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...
>>
>>     

-- 
___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
___________________________________________

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