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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