No Mach3 Index pulse with G540 controller

Hi all,
I have a G540 with four stepper axes (one a rotary) on my mini-mill connected to a Windows XP Pro (service pack 3) PC running the latest version of Mach3 through the parallel port.
All are working fine, as are limit switches on X/Y/Z axes on inputs 1/2/3.
I want to have a spindle index pulse on input 4 but cannot get Mach3 to display anything.
Index pulse is square wave 5V to 0V, one pulse per revolution, max spindle speed is around 5000.

I am using a TCR5000 index sensor, see attached pics. The sensor is working fine - I have checked it with another readout and also tried another (same type) sensor.
Input 4 is working fine also, I have checked it by swapping limit switches to it and am also now using it as a Z axis height setting sensor. So the input is working OK.

Has anyone successfully used one of these sensors for an index pulse?
Anyone got any suggestions? Things to try?

Thanks,
Davo.