Re: DMX optoisolator

On Sat, 27 Aug 2005 13:05:53 +0100, E. Lee Dickinson <lee@xxxxxxxxxxxxxxxxxxxxx> wrote:


"Christian Marg" <christian.marg@xxxxxxxxxxxxxxx> wrote in message
news:depd45$1aaq$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Hello...

E. Lee Dickinson wrote:
As DMX is balanced, is there any real reason I should connect Pin1? 

Not balanced, differential.
You still need a reference ground for the Data to reach its destination.

Isolation would need a seperate power supply (with seperate ground) for
each output. There are some RS485-Line drivers from maxim that provide for
complete insulation: They contain the needed Optocouplers and a DC/DC
converter to insulate the ground. 

I've just read an article on "differential signals," and reread the few I
have on "balanced signals" and I don't understand the difference.
Differential, according to your post, requires a ground reference.

But I fail to see why it's needed.  Can't the inverting and non-inverting
signals reference off eachother?  That's what they do in, for repeating
example, Krystof Nys's design.  Data+ and Data- drive the input
optoisolator, and ground appears unconnected.

What's the point of ground in a differential signal?




There's more than one issue, actually isolating the input or putput from ground in a high frequency is realatively difficult & expensive, isolating a DC reference is really expensive, secondarily you normally want to ground both ends of the screen in order to minimise the electrical noise in the system. If you're isolating the outputs in order to minise the effects of ,for instance, apllying mains vaoltage to the earth lead then the Pulsar solution of wiring two 5W 6V Zeners back to back across the driver, two 12V Zeners to ground & two 50Ohm PTC Thermistors in series with the line is bulletproof & much cheaper than proper isolating transformers & power supplies.
.

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