That other DIP8 opto is also a 6N137, makes sense as although DIR won't change state as often as PULSE, there are setup times to be observed. Visual checking of the little white iso-optalator proved fruitless (hidden tracks) so I resorted to a DMM to bleep it through, and it appears to be connected to the enable input, albeit via a resistor or two. Peering underneath the board however there's another one however which I imagine does the alarm output, both those pins show >2Meg to ground, so are likely floating.