Ok; the KX1 has a little board titled "XMT-SK".
Have not powered on my mill yet, so this is a guess from taking the circuit apart.
From looking at it, with the transformer on the top left, the left side connector, numbered 1 from the top to 8 on the bottom:
1 - 110v input, load (black) wire;
2 - 110v input, neutral (white) wire;
3 - +5v from 7805 voltage regulator;
4 - ground from 7805 voltage regulator;
5 - optocoupler LED, cathode in 6N137 (not connected on my unit)
6 - Forward/Reverse; tied to cathode of phototransistor (EL817)
7 - grounded on my unit to pin 4 - tied to cathode of phototransistor (EL817)
8 - to the ATMega8. Did not follow the pin; goes first through trim pot.
Anyway, my G540 is still in the mail; but in case anyone else wants to know about a "XMT-SK" board; the above is my current take on it.
Oh - and I found a label on one of the steppers; the X/Y direction are 3.0 amps, in case anyone is wondering. No label found on the Z axis yet.