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I noticed the MJP technical manual says there's 9V on pin 3 of the keyboard connector, but I measure 12V there. I'm assuming that it's just a mistake in the document, but on the off chance that my NABU is weird or I'm making an error in measurement, I'm curious whether anyone else has checked this voltage.

In December before I went to my cabin, I was probing the keyboard connector to connect an arduino with a rs422 shield like you had done. And I also noticed 12v - but I didn’t check with load. There’s a massive resistor raised on the pcb from what I recall. I wondered if that’s part of a divider to get 9v, i that case it would need load for a correct measurement.

If I load it down with 300mA, it drops to 9v. I'm tempted to measure the current consumption of the keyboard, but at this point I think I'll go back to the actual project that I was in the middle of.

From the heat generated on the keyboard, it might have a similar current draw