Hi Andy,
The USB-6218 has static digital I/O. This means that the switching is not hardware-timed. The timing is system dependent (i.e. how fast your computer is, how many other processes you are running at the same time). The accuracy of the PWM will also not be deterministic (i.e the pulse width may vary if your operating system is running other processes simultaneously). I would expect that you can switch faster than 5 times a second though. For hardware timing and greater accuracy consider using a counter instead of a digital line.
I recommend testing a simple PWM Application by connecting your digital line to an analog input so that you can use a test panel in Measurement and Automation Explorer (MAX) to view the output signal.
Here is a simple example (LabVIEW 8.0) that I have used with a USB-6008. It is set to output 100 points per period with a percentage low or high depending on your specified duty cycle. The output is set to 1 point per ms (or one period per 100ms). You will need to open the DAQ Assistant to configure it for the digital line on your USB-6218.
Regards,
Jennifer O.
Applications Engineer
National Instruments
