Another solution if I am understanding you question right is to use one of the reference positions.
Just set an unlimited movement on all the joints you don't want to monitor. For example 0.000 +/- 180 assuming that joint has 360 of total movement.
Then set your range on the joint you are looking to monitor.
Put the DO you want triggered in the Signal Definition field.
This way you don't have to use BG logic, but the output will always be active.
Of course if this range changes, position registers and BG logic are a better solution.