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Bruce in Alaska wrote in news:bruceg-
:

a lot of the original NEMA stuff used OI's as input devices just so they
wouldn't have to deal with Ground Loops on the data Paths. It is very
easy to drive the LED side of an OI with a few milliamps at 3 volts from
a considerable run of wire. Since it is digital, you just need enough
to cause a transition in the OI's receiver to cause data to flow. It
also makes driving parallel receivers, easy, by just building the driver
side with enough power to drive multiple LED's in parallel.

Bruce in alaska
--

A 12 cent differential MOSFET-input op amp draws ZERO power and you could
parallel a infinite number of NMEA devices with just one load resistor on
the end device.

But, alas, that would eat into profits of the $10,000 units and ****
everyone off.....