> 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...
> 

I have done something similar and the biggest problem was voltage drop on the 
power line. Not so much in the cable but more in the terminals. The first 
version used two terminals, one for input and one for output that was connected 
in parallel on the circuit board. In the next version I used one single 
terminal and connected both the input and output cables in the same terminal 
which improved it a bit. Each node consumes about 50mA max so it is a bit more 
than yours though.

Another difference is that I don't use CAT5 cables but standard industrial 2 
twisted pair with shield. Cable area at least 0.5mm2. The shield is clamped 
directly to the groundplane on the circuit board, no isolated transceivers but 
good transient protection on the RS485 lines.

An installation normally uses between 10-50 nodes over a length of a couple of 
hundred meters. A PC with a simple RS232/485 converter is used as a master. 
There are a couple of thousands of these boards used all over the world and 
once everything is up and running they seem to run forever.

/Ruben







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