using 7i47s analog output as analog input in frquency drive

Trying to connect the analog output (pwm ) from 7i47s to a frquency drive with 15 volt input / ground and slider .
when monitoring pwmgen output on hal scoop we will have an output when set pwmgen to enable, when changing the spindel speed
on linucontroller, we do get a different output on our scoop, but on the output pins we don't measure any voltage.
We have 15 volt on A+, 0 volt on A-, but no voltage on the slider .

Are you measuring the voltage relative to A-? (the output is floating so must be measured relative to A-)
To get an analog output you need a reasonable PWM value (say 50% of full scale to test), a proper PWM frequency
( The 7I47S manual suggests 5 KHz) and the enable low at the interface pin. This would typically be GPIO18
if the 7I47S is on the first IO connector (GPIO 00..23)