mutiple pulse modulation

[James3]

Hello,

I am looking for a way to create multiple PWM signals using a NI-6254 DAQ and labview 7.1. For now I would like to create 8 unique digital signals that I have control over the pulse width. I noticed this card has 2 counter outputs and this works well, however I need 8 signals. Is it possible to use one counter signal and then route that to 8 pfi pins? After I do this would I be able to manipulate this signal independantly. By this I mean each of the eight signals could have a different pulse width. If this is not possible would anybody have an idea on how I could accomplish this task?

James

[James3]

I forgot to mention that I have MATLAB at my disposal, maybe I could create a script to create a pusle width modulated signal?

Just throwing ideas around. As of riht now I am not too concerned about accuracy.

[Tom_W_[DE]]

Hi James-

It's not possible to route the counters to multiple pins and then control them independently- you only have two sets of counter hardware at your disposal, so you'll only be able to create two PWM signals at one time.  Your best bet would be to use a card with eight counters such as the PCI-6602.

You could use the correlated digital output capabilities of your board to generate a PWM signal, but the updates to freq/duty cycle would be non-deterministic. 

Hopefully this helps-

[James3]

Tom,

Thanks for the reply. That's what I thought, I didn't know if there was a clever way around it. I can't get a different card because this is for a school project so I have to think of a way around this. Currently I am thinking about using a LMD1825 motor driver. I think that chip and the DAQ will work well together. According to the data sheet one can use digital signals to control the pulse width. I'll post my results after I get the IC in and play around with it.

Thanks again.

James

[Tom_W_[DE]]

Hi James-

The external IC may be the quickest way to reach your goal- please keep us posted on your progress and if you have any additional questions.  Best of luck

[James3]

I am looking now for a way to read ttl voltage levels and reconize them as boolean values on an analog input. Is this possible? So what I have are digital signals, but I want to read them on a designated analog input on the DAQ and then convert the signal to a boolean value within labview and analyze that data. Has this been done before? Is this something worth trying?

Thanks for the fast responses!

James

[Tom_W_[DE]]

Hi James-

Is there a reason why you don't want to use the digital input lines on your device to accomplish this?

[James3]

Yes, I would be using the digital signals as outputs.

James

[Tom_W_[DE]]

Hi James-

The quickest method I can think of is to just specify a logic threshold division and then check to see if you are above or below that level.  I have attached a VI that shows how this might be done; you may want to add a VIH and VIL (rather than a simple threshold) check in order to define a deadband in your AI signal.

[James3]

Tom,

Excellent! I donnot have access to a machine with labview until later today so I cannot play around with it until then. But I'll let you know how things go, from what you say it sounds like it would work for what I am trying to do.

Thanks again!

James