Simple enough . . . . you wire up a small 12 volt control relay - + connection from the relay goes to the Aux + power output the relay - goes to the output in question.
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Programming:
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Locations 46, 47 and 48
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46 segment 1 set to '9' (AC Fail)
47 segment 1 set to '0'
48 segment 1 set to '0' (Follow)
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Segments 2-4 of 46 and 47, and segment 2 of 48 and 'don't care' if you use no other outputs.