Spindle control from GRBL 1.1

I have an xPRO V5 running GRBL on my 3040 CNC router, with the BLDC spindle controlled by the PWM output going to a Novusun NVBDL+ spindle driver (the no-Hall effect version).

My problem is that I cannot make the spindle stop. I have it connected so that the EN output from the xPRO goes to the CW (run clockwise) input on the spindle controller, and the PWM output of the xPRO going to the PWM input of the spindle controller.

On power up, the spindle immediately starts running full bore (12,000rpm). the mode settings on GRBL show M5, ie spindle disabled. As soon as I issue a M3S10000 (enable spindle) the spindle changes speed to 10000 (or whatever you request) and from then on all speed changes are effective. BUT, M5 does nothing. The EN output does change state with M3/M5 (as measured). The PWM average voltage varies with speed commanded. But when the spindle is disabled, there still appears to be a voltage on the PWM.

So, how do I get the spindle controller and GRBL to play nice together? Has anyone successfully connected a spindle via GRBL to the PWM of a Novusun spindle controller?

thanks
Rob