Working with RS485

[dhouston]

If you also read the DMX512 specification you will see that it is essentially a variation on RS485, running at very high speed and with the master constantly repeating commands (or state table). Cables that work for DMX512 should also work for RS485 although perhaps not at the extremely long distances possible with RS485. Before interconnecting grounds you need to measure the difference in potential (i.e. voltage difference). If more than 7V, connect to local ground through a 100-ohm 1/4W resistor. This is detailed in the official RS485 specification. You might also do a web search using "Jan Axelson RS485". You should find a few magazine articles by her, including one which shows using a 555 timer chip to automatically reverse the line after a transmission. She writes very clearly.

[The Master]

Just a thought, All of these lights will almost certainly be run from the same AC socket. If not then definately in the same building. Each one will have its own transformer though. Is that what causes the ground difference?

The total distance between the master and the final slave wont be more than 50 meters.

Im not quite sure i understand ground difference properly. Each device has a transformer so its isolated from the mains. i cant see how i could get a reading accross 2 grounds unless im thinking of something different. Im sure this is explained in the rest of that document though

[dhouston]

• http://en.wikipedia.org/wiki/Ground_loop_(electricity)
  http://www.epanorama.net/documents/groundloop/
  http://www.trinitysoundcompany.com/grounding.html
  http://zone.ni.com/devzone/cda/tut/p/id/3394

• http://en.wikipedia.org/wiki/DMX512-A

[The Master]

Ive read all of those documents. I understand the one about DMX but the ones about ground loops are a bit confusing. From what i can see i shouldnt have a problem because all devices are isolated from the mains using a transformer for power and opto isolators for the triacs. There are earth wires going inside the lights but they dont connect to anything since the lights are made of wood. Each light has IEC in and out sockets so all of the lights could be run from a single socket. In practice i will probably have them in 2 groups (left and right) but no matter how many groups i have they will all plug into an extension so they will all run from just one wall socket.

I wouldnt have thought i would have any problems because of all the low voltage stuff being completely isolated from the mains and the short cable distance but i have noticed problems with my speakers before. If i connect a laptop to my mixer at the same time as my speakers i get very loud buzzing until i press the ground lift button on both speakers. That happens with the laptop, mixer and speakers all plugged into the same extension