[SOLVED] Power to a microcontroller network... (rs485)

[henninggs]

Hi.

I'm planning to make a network of microcontrollers as part of a home automation system. I was thinking of a rs485 network. As I understand it, for a full duplex rs485 network I'll need 3 twisted pairs. So if I use a cat5 cable I'll have one twisted pair not in use. Is it a good idea to use this pair for distributing power to my nodes ?

if so:
1. What would be a useful current to use, 12v ? (Max distance from the master to a slave is 150 feet. I'll have up to 32 slaves-nodes)
2. How do I regulate the voltages in a node to something useful for my PIC's ?
3. Any idea of how this could be done ? (Schematics, hints, explanations, links)

My nodes will not use much power they will primally be used for readiong some sensors and send the data back to the master.
Thanks for any replies.

 

[bjuric]

Full-Duplex RS485 requires 4 wires (2 twisted pairs).
The DC resistance of a Cat5e cable is about 3 Ohm per 100 feet per wire, so you shouldn't experience significant voltage drop.
12V should be more than enough for a bunch of sensors, than just use a 78L05 to regulate 5V for your MCU.
Depending on consumption, you might even be good with 5V and larger caps in the nodes, but it's a gamble.

Google for "Guidelines for Proper Wiring of an RS-485".

 

[henninggs]

Perfect.
Thanks a lot bjuric.