In reply to:

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Just remember that the +12 volts on the serial line and the remote Amp control line share the same source, so can only sustain 1 amp between them.
For most "correctly done" installations this is never going to be a problem but it is something to watch for.

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Ah, thanks. I'm actually closer to getting the board finished than I had thought. (Simple designs rock.) The screen does NOT suck that much power, so I will go ahead and use the serial DTR line, with a standard DC plug. When something is in the plug, the +12v/DTR is cutoff from the DB9 connector, and used from the plug. Otherwise, +12 comes from the DB9. Perfect.

Me.