For what it is worth to people discussing DMX/RS485... I solved the DMX 
isolation problem with a Maxim MAX1480ACPI chip.  28 pin DIP, 
transformer inside of it, and at $11 for qty 100 it is always the most 
expensive part of the design.

I've been wanting to do something about this, as in, trade a large 
expensive single chip for a few of something cheaper.  However, all the 
other isolated RS485 stuff I've seen is data only.  Some datasheets give 
sample mini isolated dc/dc circuits using unavailable magnetics. 
Nothing compelling.

In my designs there will always be a micro that can generate a PWM to 
run a transformer (or whatnot), and there could be some sort of isolated 
feedback to regulate the PWM on/off so that the isolated side generates 
a voltage within a decent range.

Dwayne - would love to see your circuit as well...
(<http://www.celestialaudio.com/ca_dmx_32_V2_nfet/index.html> is the 
board I use the maxim part on, just FYI, and have used it in other 
designs as well.)

Cheers,
J





Mike Harrison wrote:
> On Tue, 6 Oct 2009 13:29:55 -0400, you wrote:
> 
>> Dwayne Reid wrote:
>>> I have been doing something similar for several years now to receive
>>> DMX lighting data.  In my case, I use a tricky little homebrew DC-DC
>>> converter to generate 5V @ 50 mA which is then used to run the
>>> receiver chip and an opto.
> 
> Funnily enough I've recently been thinking about how you might be able to do isolated DMX receive
> with no power on the RX side. 
> A few ideas I had - not tried any yet....
> 
> As DMX is RS422, you potentially always have power regardless of data content & duty cycle
> 
> a) A low-power oscillator, maybe low tens of MHz, simply powered from the data lines, coupled
> inductively or maybe capacitively through the PCB, so no transformer. The RS422 spec states a
> maximum number of receivers, so as long as you stayed within that limit it might be doable in-spec.
> 
> b) most DMX devices (especially those you want to save every penny on) only use a few channels, so
> the data rate out of the interface can be somewhat lower than the 250K line rate - this may have
> possibilities to reduce the bandwidth of the isolation components to save power/cost if you can do
> the 250k receive at low enough power. Not sure how practical this would be as you'd still have to
> receive at 250K and do the byte counting for addressing. .
> 
> c) I saw an interesting circuit idea in a mag recently for isolating an SPI port - driving a pulse
> transformer with very short pulses (40ns or so), derived by driving each side of the primary with a
> slight delay, using a couple of HC04 gates. This converted the input pulse edges into short +ve and
> -ve pulses  The receive used a bus-hold latch arrangement, again with a couple of hc04's, withits
> state flipped each way by  the short pulses.  As the pulses are very short, the avarage power draw
> is very low, and may be doable with power leeched from the RS422 lines & some capacitance. 
> 
> 
> 
> 
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