Re: DMX optoisolator

There's more than one issue, actually isolating the input or putput
from
ground in a high frequency is realatively difficult & expensive,

What do you mean by 'high frequency'? Real HF signals travel around on
co-axial cables, using ground as a reference.

isolating
a DC reference is really expensive,

This needs some explanation. Off the top of my head, I can't see why.

secondarily you normally want to
ground both ends of the screen in order to minimise the electrical
noise
in the system.

This is one thing that you don't want to do. It allows for circulating
currents, and I don't know what else. Cable screens need grounded, yes.
But at one end only. This has been standard practise in the BBC since
long before I joined, in 1965, and has caused me no problems since.

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.

You said it, I didn't. "Proper isolating transformers and power
supplies".Sure, you can bodge it with Zeners and PTC thermistors. I
borrow a phrase from Anne McCaffrey: 'Do it right, or don't do it'.

.

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