I thought of doing the same kind of thing. There are plenty of extra wires in Cat5 cable. The only issue I could think of is possible corrosion of dissimilar metal contacts using DC, so I was leaning to towards low-voltage AC. The RJ-45 jacks are usually rated in amperes so there would be no problem with current-carrying capacity. Also daisy-chaining means that a failure anywhere brings down everything beyond the fault. /\/\/\/*=Martin ----- Original Message ----- From: "Kenneth Lumia" To: Sent: Monday, December 17, 2001 4:21 PM 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 > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu