Hi,
This would really depend on the hardware you are using and what it supports. Generally counters are used to generate pulse trains - they are available on all multifunction DAQ products (save the USB-6008/9). If you're using DAQmx (which I recommend) then you'll wan to look for an example called "Gen Dig Pulse Train-Continuous" If you are already using your counters for something else, then if you are using an M-Series product you can use correlated Digital output to generate a PWM. You'll want to start with an example called "Cont Write Dig Port-Ext Clk" - you'll need to supply a clock from somewhere else though. If you're using an E-Series or static digital board, you could use SW timing to generate a pulse train but the frequency and duty cycle would not be that accurate. There are DAQmx shipping examples for all of this for LabVIEW, CVI, ANSI C and .Net - please post your HW, OS and programming environment and we can point you to an example to get started.
cheers,
Andrew S
