RS458 Cabling

I have a piece of equipment that uses RS485 with RJ45 jacks and I was
wondering if I needed special cabling with only 4 of the wires phyically
connected in the jack or could I just use regular CAT5 Ethernet cabling
with all eight wires present? The specs that I have for the cable only list
4 wires, but they are in the same order and place as a regular CAT5
Ethernet cable. The cable will go between two like devices over RS485
connected by RJ45 jacks.

On Fri, 12 Jun 2009 12:19:27 -0500, "bfranklin825"
<bfranklin@smurfit.com> wrote:

>I have a piece of equipment that uses RS485 with RJ45 jacks and I was
>wondering if I needed special cabling with only 4 of the wires phyically
>connected in the jack or could I just use regular CAT5 Ethernet cabling
>with all eight wires present? The specs that I have for the cable only list
>4 wires, but they are in the same order and place as a regular CAT5
>Ethernet cable. The cable will go between two like devices over RS485
>connected by RJ45 jacks.

RS-485 requires only a single twisted pair assuming that proper "fail
safe" termination is used (if no termination is used, you also need a
signal ground reference).

If standard CAT5 Ethernet cabling is used, make sure that you use the
wires from the _same_ twisted pair for RS-3485 A and B connections.
Using the A signal from one twisted pair and the B wire from an other
twisted pair is going to ruin the noise immunity quite quickly :-).

Paul