7i76e Analog spindle control question

I'm trying to use the analog spindle control on the 7i76e to control a speed controller that uses a potentiometer on a floating ground at line voltage. I'm using a KBSI-250D signal isolator to protect things and bring the signal from the 7i76e to the proper levels. I have configured the spindle output to 5v at full speed (as required by the controller).

Current wiring configuration: 
Ground from an unused step/dir to TB4 pin 1
5v from same unused step/dir to TB4 pin 3
TB4 pin 2 to KBSI  input +
Ground from unused step/dir to KBSI input -

Problem is, I only get 2.44v at full speed rather than the full 5v I would expect. Interestingly, measuring TB4 pin 2 and pin 3, I get 2.55v (making up the rest of the 5v I was expecting). 

I'm sure I'm missing something obvious, but I sure cant see what it is.
HAL and INI files attached.
Any ideas?
Darryl
 

You have (in the spindle section of the .ini file):

OUTPUT_SCALE = 21000
and
OUTPUT_MAX_LIMIT = 10500

This means you can only ever get to 1/2 full scale (10500/21000)

Brilliant! Thank you!
Darryl