mutiple pulse modulation

[Tom_W_[DE]]

Hi James-

Glad to help- let us know if this doesn't work out for you.

Thanks-

[James3]

I tried the VI and didn't see what I hoped. I didnt see the digital state turn high until I set the logic threshold to -6 (-5.279 after playing around with it more). Originally I had it set to 0. I am sending +5VDC and the level never changes.

 I am using dev1/ai0. I have pin 67 connected to gnd (the AI gnd) and I also have pine AI sense connected to gnd. I also tried it with and without apfi 0 connected to gnd. Do know why I see this?

Thanks again Tom,

James

[Tom_W_[DE]]

Hi James-

Let me make sure we are using the same connections:  you should have the positive terminal of your source wired to pin 68 (ai0 input) and the negative terminal of your source wired to pin 67 (AIGND).  I noticed that I did not specify the RSE terminal configuration directly in my earlier post, so I have modified the VI to force it to use RSE terminal configuration.

When you wire a DC source I would not expect any change in the indicator; you will need to test with a source that slowly oscillates above and below the logic level to see any change on the indicator.  Let me know if you're not able to see better results-

[James3]

Tom,

Thank you for taking the time toshow me how to do that. I didn't have time to look at it too much, all I did was test it against 5 and 0 VDC and that worked. I'll test it against a clock signal and see how fast it can record the data.

Happy Holidays,

James

[Tom_W_[DE]]

Hi James-

That sounds promising- let us know if you're still running into problems.

[alex_bcn@fantasymail.de]

Hi James!

Did you acomplish your aims with the programme? Seems like I´m having almost the same problem, although in my case I only have to control 4 digital Outs so that I don´t need the analog INs...

Would you mind sharing your knowledge with me? The problem is that I´m almost a complete Labview-Starter and don´t know that much about digital signal processing.

Thank you in advance,

Alex

[James3]

Alex,

I did accomplish what I set out to do. I don't quite understand what you are trying to do though. What DAQ card do you have, as I understand they are all different. I needed 8 timers, my card only had 2 so I used different hardware to achieve my goal. Let me know what exactly you are trying to do and I might have a suggestion. If you only have to control 4 digital outputs that program should be very very easy....

Best of luck

James

[alex_bcn@fantasymail.de]

Hi James,

I have got an PCI 6254 and the aim is to generate 4 (better: 6) digital rectangular signals with pulse width modulation. That is, it should be possible to adjust frequency, duty-cycle and time seperation between these signals for each one. I think that task may not be that difficult, but I´m new in LabView and DAQ so I need some hints or an example. I would be pleased if you could send me a copy or part of your progamm in order to try it out on my system.

Thank you,

Alex

[James3]

Alex,

I looked at the data sheet for your card and it says that you only have two counter timers. If you have the option to purchase a card with more counter timers I would recommend that. If not then you could use software to compute your PWM signal, but this is not a good way to accomplish what you are trying to do if accuracy is an issue. What I did was I found an IC (LMD18245) that created a PWM based on an RC pair and a binary number. What exactly are you using the PWM signal for?? Keep searching the net and using the Labview software. Good luck, hope this helps.

James

[Travis G.]

Hello James,

The best way to generate digital pulse trains for PWM applications is using the counters on your DAQ device.  Alex is right when he states that your PCI-6254 device has two counters on the card, so you are limited to generating two simultaneous pulse trains with this device.  If you are looking for a device with more counters, National Instruments sells dedicated counter/timer devices that contain up to 8 counters per card.  Take a look at this comparison chart to see the different channel counts and specifications for the counter/timer series devices.

You can use the same program to implement PWM using the DAQmx driver with your PCI-6254 M Series device or with one of the counter/timer devices.  Here is an example program that shows how to change the pulse specs of your pulse train on the fly in terms of duty cycle and frequency.  I hope this helps!

Regards,
Travis G.
Applications Engineering
National Instruments