Phantom power on RS485 network

RS485 is a pretty specific electrical specification that doesn't include phantom power, so it wouldn't be RS485 when you were done.

You may be better served by investigating how the model railroaders do signaling over track power -- it's low voltage, capable of moderate power and data rates, it works in high interference environments and it's cheap.

You'll have to google around to find out if you can get technical specifications, though.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott scrobe on the papyrus:

Thanks Tim, I hadn't thought of that. There have been a few articles in Elektor over the last few years on the Marklin system. I'll have a look.

--
John B

You have not said how many inputs and outputs each box has.

The most general solution (that I would do) using three wires is:

1) Power feed

2) Ground

3) Bi-directional TTL serial communication

Put your choice of microcontroller in each unit and feed as many inputs and outputs as you like. Since this is direct wiring you won't need error checking or anything fancy for the protocol.

Luhan

You haven't said how much data you are sending, this might be useful, and are you looping through, box to box, and is it bidirectional? it might be possible to FM a 40KHz signal for data and use transformers for phantom powering. How much power do you need, anyway? Do you need to put baseband audio into the system as well?

martin

martin griffith scrobe on the papyrus:

There will be no baseband audio and the data rate will be very low. The idea is that there will be several boxes around the stage. Each will have two buttons and two LEDs (red & green). The master station will monitor all boxes for button presses and flash the appropriate LED on the box where the button was pressed. All LEDs can be cancelled from the master station which will have clear buttons and an LCD display to indicate which box was activated. I am planning on using ATmega8's in all the boxes and a serial protocol using ASCII messages. The protocol will require two-way communication on the network and the network will probably be around 75 metres in total length. I am thinking of RS485 as it doesn't radiate emissions as badly as an unbalanced system and there are microphone cables running nearby.

However you have given me food for thought and I could use a system similar to 1-wire. A pull-up at the master station and all USARTs could connect their TX pins to the bus through open-collector drivers. RX pins could simply hang on to the wire. I don't know if this would system would be capable of 75 metres, but I'll kludge something together and see what happens.

Thanks to all for your ideas so far.

--
John B

Sounds like a reasonable plan, but split the pullups at both ends. Driven with an NPN transistor to ground, or the equivelent ic output, I would go with about 200 ohms pull ups, for a total of 100 ohms. Then a slow data rate - say 1200 baud, because you are not sending much data nor needing especially fast response times. I double that this will induce much noise into nearby, ballanced, shielded, microphone cables.

Good luck, Luhan

Only one gripe,cheap low end DMX uses 3 pin XLRs, Its probably too easy for a beginner to cross the DMX cables with your signaling cables, I'd change the connector to something a little less "standard"

Steve Roberts

Rinnai uses s.t. like that with their "Infinity" hot water heaters. We have three control units attached in a Y network to the same

2 junctions inside the box, using probably 20+metres of figure-8 cable. The data rate is low, but the LED displays in each panel are powered by the lines, which seem to be non-polarised - I'm guessing a bridge rectifier inside each panel. It'd be interesting to scope it and see what they're really doing.

X10 uses the AC-network to transmitt it's signals. PoE can use all four wires to power devices and still keep 1Gbps data asfaik. Etc.. (google..) It's all about seperating DC & AC.

How many boxes will there be?, Led-only box seems kind of light for a scene ;)

Two or even one io pin of the mcu (atmega8) could be used to setup a very cheap transmission system. A capacitor in series with the signaling pin should keep data signals seperate. A coil in series with the mcu powerpin ought to supply power. Care would have to be taken to resonance and stability of volt level. You have to verify this stuff on powering the mcu thoe :) For RS-485 operation additional driver ic is needed (costfactor).

Given l=75 meters, cable velocity vf=.42 (?), m=5x propagation marginal

1/(l /(vf * c)*m) 1/(75/(.42 * c)*5) = 336 kbps possible ..?

Just a quick guess..

Maybe some rc filter could fix that by slowing rise and fall times.