Page 7,i.e. Figure 4, specific to 6512, of the NI651X Specification manual shows the connector pin out.
My question is related to pin 9, (P0.COM (P0.GND)). Is this pin tied to ground? Or is it the common pin for all port 0 pins.
So in my case if I were to connect +28V to pins 10, 11, and 12 and +28V return to pin 9. When I control P0.0 (pin 1), I would swtich pin 1 output between +28V and ground?
P0.COM is the common pin for port 0 pin. You should connect the ground of your external power supply to this pin, however the pin itself not tied to earth ground.
To answer your specific question, if you make the connections you mentioned, you will be switching pin1 between +28V and "+28V return".
Hope this helps.