Generate two alternating PWMs with one counter of 6025E

Question

I am using LabView 6.0.2 with a PCI-6025E card. My requirement are to put out a pwm signal with 250Hz. The problem is, that for example, first a PWM with exact 100 pulses of duty cycle 1% is needed, after that exact 5 pulses of 10% duty cycle. These two PWMs are repetetive without any big pauses between. I tried this with Pulse Train Generation with "Changing Pulse Specs.llb" from the ni libraries, but the PC based on windows is not able, to do the PWM changes in these exact time slots. Note: The second timer of the card is used for another item.

Answer 1

Unfortunately, you won't be able to do exactly what you want with a single counter. You'll need to use additional hardware. Here are some possibilities:
1. Analog Output:
A 1% duty cycle at 250 Hz requires a minimum of 25 kHz update rate, but your 6025E only goes up to 10 kHz. Your board is also limited by having only 1 DMA channel for Data acq buffered operations, and it may already by used by your other counter's operation.
If you had an analog board with faster output, you could try to create a buffer with 0 V and 5 V values representing the desired finite pulse train. However, if the device you're controlling with PWM requires TTL edges, the rise and fall times of your DAC output may still not be fast enough to fool it.
2.
PCI-6601/6602 Counter/Timer:
You could use two counters from a 6601 to generate the finite pulse trains you describe, but there would need to be a pause between the the two duty cycles. The pause is where your software detects the end of one finite pulse train and reprograms the counters to generate the other one. The pause might likely vary from 10 to 100 or more msec.
If that variable dead time between pause trains is acceptable, the 6601 is the cheapest solution I know of for today's application. If you need reuse for future apps, you might want to consider the 6602 for double the price, but a bit more than double the capability.
3. PCI-6533/6534:
NI's timed digital boards would allow you to define the pulse trains as a one-bit digital pattern that you could generate continuously. I suspect the 6533 would suffice for the app you describe.
Please double-check these hw suggestions with your local rep before ordering. (S)he'll know more than me about
your present-day app and potential future needs. Good luck!
-Kevin P.