How do I connect 5V index pulse to UCBB?

[DavoH]

I have my mill spindle controller connected to the UCBB OK but don't know how to connect the index pulse to it.
The index pulse is a 0V -> 5V square wave.
I can see the pulse on the UCBB input terminal it's wired to but its UCBB LED doesn't light up and software does not display any speed and the diagnostic page doesn't show any Index pulse on that pin, or the other pin I tried.
Sorry, I can't remember what pins/ports I tried as I'm not sitting in front of the machine right now - but they are both input pins.

I've wired up my limit switches OK (that's shown how to do it in the manual) but don't know what to do with the Index pulse input - I used the same port as the limit switches.
I am mystified by the fact that all input pins have a negative and a positive designation.
I tried connecting the negative one to ground but all that did was to reduce the index pulse voltage somewhat.

Secondary question:
I want to buy a Differential Rotary Encoder to mount on the spindle so I can control speed from software.
Top speed of the spindle is 6,000 rpm.
How many pulses per revolution of the encoder should I get? There are so many to choose from!

Hope you can help.
Thanks,
Davo.