I was on my local supplier's website yesterday and dropped a pack of 4N25s in my basket.
I notice, looking at this, that the 4N25 also brings the base out to a pin. I was wondering why? I imagine it would be useful for testing for example, where the base is controlled manually without having to energise the HV side of things. Any thoughts?
(My link also shows that the OP is exactly right: a series resistor on the input and a pull-up on the signal.)