It’s been bugging me for a while: There are two ports on the electronics cabinet which aren’t connected to anything. They’re about midway between “I” and “D” on the diagram (see below).
What are these for?
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That brings to mind the original 1966-1969 Star Trek series, with whole control panels of unlabeled switches and indicators… Maybe it’s a “need to know” situation… 
They look awfully similar to the existing switch and motor inputs of I add H, just smaller 
In pure speculation, I can only hope it’s for some unknown accessories. Fourth axis/indexer maybe? Being motor output and switch input? A man can dream (not that it matters all that much to me, I don’t have an HDM 
)
If I end up owning one this year, I’ll crack that sucker open and let you know if it’s connected to a driver IC or something on the main board 
Edit: Dang, the exposed heat sink might indicate there’s not an extra driver squirreled away in there. With four driver sinks, that already accounts for z,x, dual y. If that’s the case, I’m out of ideas