I would recommend configuring your program like the example program, Real-Time PID Control.vi, that ships with LabVIEW. You can find it in LabVIEW by going to Help->Find Examples and type "PID" on the search tab. The Real-Time PID Control.vi shows a good way to design your program for a single point in, single point out application.
If you're aiming for a 20KHz loop speed, you may not be able to attain this with the PXI-8175, assuming you also have a normal priority loop running to handle the communication back to the host computer. To time your code, you can use the NI Timestamp Code Time.vit template that comes with LabVIEW 7 Express. The PXI-8176, benchmarked for sinlge point PID loop rates up to 40 KHz, may be a better solution for you application.
It is
also a good idea to disable the USB port and to install only the RT drivers you need. For information on how this is done, follow this
link.
Please let me know if you have further questions.
Regards,
Kristi Hummel
Applications Engineer
National Instruments
