Seal Lee,
The 7344 sets the PWM frequency by downcounting an onboard clock by a user selectable factor. 50Hz is not one of the standard frequencies available. You could get a frequency of 50Hz by using the external clock feature documented in the Flexmotion Software Reference Manual, however I think is probably not a good solution.
http://www.ni.com/pdf/manuals/321943b.pdf
As I understand it, RC hobby servos require a 1-2 ms control pulse repeated every 20ms or so (50Hz). A 1ms pulse represents one end of travel and 2ms represents the other end. The 7344 PWM duty cycle is specified as a value from 0 to 255, where 256 equals 100%. A few simple calculations will show that this gives roughly 10 distinct positions for your servo, which is not very good resolut
ion.
http://www.epanorama.net/documents/motor/rcservos.html
You will probably be much happier using a dedicated counter/timer board such as the NI 660x or even an MIO board, such as any of the E Series DAQ cards. Any one of these will provide a control pulse with far better resolution than any RC hobby servo available, thus the board will not be a limiting factor.
Best Regards,
Brent Runnels
Applications Engineer
National Instruments
