RS485 Phantom Power


I am working on an eight channel theatrical cue light system.

I need a way of connecting three individual LEDs and a button at Point A to an Arduino at Point B. Point A and Point B will be anywhere from 20 to 200 feet apart, and they must be connected via a single three wire XLR cable. Wireless will not be viable where the system is deployed. I wish I could use Cat5, but sadly that's not possible either.

I've decided on using RS485, but am unsure of how to provide phantom power to the devices on the network. There will not always be a power connection by every device, so phantom power is necessary.

How can I create a system that provides both power and RS485 data through three wires?

Thanks for your help!

Well you could have two of the wires be power lines, and the other one be a data line. The only issue with this method is the clock. All the clocks will need to be in sync otherwise you will get wrong data. However, if you can maybe change the cable from a XLR to maybe a 3 Channel 1/8 or 1/4 inch audio jack, this way you can have the two power lines and a data + clock lines. If that is not an option then look at this site. Lowes_cables

I did power and signal over two wires once. I had a local cap and a diode at the far end and transmitted serial data on the power wire by shorting it to ground. The cap held the remote voltage up long enough so the circuitry stayed running.

This used async comms though so no clock was required but you have 3 wires so it would still work.

Also if there are large current draws is might be a problem but super cap would help.

If you have to use RS-485 you might be able to power the remote system through two shottky diodes from the A and B signals. It will depend on how often you need to send data and for how long.


Well you could have two of the wires be power lines, and the other one be a data line.

And how would that be RS485?

The obvious solution is to use DC-balanced differential signalling on the RS485 then
you can isolate the signals with pulse transformers just like ethernet and superimpose
your power.

You'll need a balanced encoding (manchester?) so couldn't just drive it from Serial.

If you don't actually need the 10MHz+ signalling bandwidth that RS485 would provide,
how about just using two wires for power and one for 9600 baud TTL serial?

Thanks for the quick responses, guys!

I've looked into the TTL serial method, but I am very concerned about interference in the lines creating noise and providing me with false data bytes.

For that reason, I've decided on using balanced RS-485. I will be using two LTC1480 chips as seen at Gammon Forum : Electronics : Microprocessors : RS485 communications. The master device, an Arduino Mega, will be polling the slaves, ATTinys, as suggested by Nick Gammon at cue light system, half duple serial over long distance and power over the wire - Project Guidance - Arduino Forum. I understand how to do this much, but I do not understand how I could inject power into one end of the system, and use it at the other end to power the RS-485 chip, the ATTiny, and an LED (only one LED will be active at a time).

How exactly would I go about superimposing the power into this system?

Also, those "shottky diodes" might be an option... Would it work if small bits of data were constantly being transmitted over the line? The thing is though that the data will need to switch direction half the time, for the slave to respond to the master if any buttons were being pressed. Is this possible with that method?

Thanks for the help everyone!

Bidirectional signal and power on three wires over long distances. That’s a big ask! Keep in mind for 200 feet that you’ll probably want to use high(er) voltage on the line and regulate down at the far end. Use the most efficient regulator you can. Drawing heaps of current will make matters worse.