PDA

View Full Version : RS-485 Transceivers, to isolate or not?



snarfer
08-06-2008, 04:41 PM
I'm working on a design for an LED dimmer, and I wonder if you guys could give me some advice on the issue of whether to fully opto-isolate the DMX transceiver. I've seen a lot of different approaches taken.

Some people say it's best just to stick an SN75176B in a socket and if it blows, just replace it, it's only 25 cents after all.

Other projects, mostly DMX-USB converters, have full isolation with 5v-5v isolated DC-DC converter and so forth. I guess the concern is with protecting the computer there.

So I'm wondering what standard accepted practice with an LED dimmer that runs off of a DC supply is. My device might well be attached to professional theatrical equipment at some point, and I'd like to adhere to best practices, but if I could use something like one of the tiny Intersil chips that can run off a 3v supply that would really save a lot of circuitry and board space.

Thanks for any help!

David

DynamoBen
08-08-2008, 07:12 PM
I'm sure several ppl will jump in with their opinions soon. Until then I can tell you professional theatrical lighting manufacturers typically opto isolate the transceiver from the rest of the circuit (6N137), on DMX receiving devices. It’s not all that often that you see the DC to DC converter due to cost and board space.

On opto splitters and transmitters like lighting consoles you will typically see an opto and a DC to DC. The idea is to protect the expensive console from harm that might come up the data line.

After working professionally for many years I saw my fair share of equipment get damaged. Usually the equipment was “killed” by lightning via AC power. Rarely did the damage from the data line. In the few cases I did see data line damage I just swapped out the transceiver (left the opto untouched) and everything was up and running again.

That’s what I know and saw, in the end it’s up to you.

snarfer
08-09-2008, 03:30 AM
You know it was kind of mystifying me why people talked so much about these problems that in 12 years of running DMX and lighting boards on movie sets I can't say I've ever encountered. Sure every once in a while something didn't work so we sent it back to the lighting house and forgot about it. Also sometimes the Strand equipment wouldn't talk to the ETC and we had to put in some opto-splitters. But never once had an actual blown RS-485 transceiver.

But now it makes sense. These problems really come up more with theatrical installations and and traveling shows, where you might have thousands of channels or be outside in a tent and vulnerable to lightning.

I had been looking at some integrated options like MAX1480. But now I think I will just do it the old-fashioned way with 6N137s and a DC-DC converter. It might be a bit clunky, but at least it will be bulletproof. Thanks.

DynamoBen
08-09-2008, 11:07 AM
Also sometimes the Strand equipment wouldn't talk to the ETC and we had to put in some opto-splitters.


I used to work at ETC, this issue is something to be aware of. ETC and most other manufactures float Pin 1 (DMX shield) on all receiving devices, and tie Pin 1 to AC ground on all transmitting devices (consoles). Some Strand stuff isn't setup this way which can cause communication between the two to fail. By dropping in an opto splitter inline you isolate both sides and they magically work together.


These problems really come up more with theatrical installations and and traveling shows, where you might have thousands of channels or be outside in a tent and vulnerable to lightning.


Actually I saw these issues most often in permanent installations. The traveling shows never had things hooked up long enough for it to be an issue. ;)