PCI-6251 reference analoge edge trigger not work with two channels?

[detech]

I use this PCI-6251 card for acquiring data from two input channles. I need to use reference analoge edge trigger. the trigger signal from chan 1 and acquire data from chan 1 and chan 2. However, the error is this card can not take data from two channels with reference analoge edge trigger. Please check if this is the limilation from PCI-6251. If so, can you suggest any other card can do this job? The regular scope can trigger from one channel and acquire data from many other chaneels.

Thanks

Liming

[DianeS]

Hello,

Your problem is due to the fact that you are trying to use a single channel both as a trigger and as an analog input.  You can't do that.

You must specify a different channel to use as the input trigger.  So, for instance, if you want to collect data on AI1 and AI2, use AI3 (or AI4, or AI0, or whatever) as your trigger input.

I would advise you to look at the examples that ship with LabVIEW.  Click on "Find Examples".  Then go to "Hardware Input and Output", "DAQmx", "Analog generation", "Voltage".  In there you will find some VIs that come very close to doing what you want to do.  In particular, look at "Cont Gen Voltage Wfm-Int Clk-Analog Start.vi".

d

[detech]

Thanks. I did not understand this point.

Further, if I use another chan as external trigger, I can input a DC signal say 10 Volt. Here we still have to set trigger level, is this level the same as 10V or less 10 volts or something else?

The reason is when I use scope to acquire signal from chan 1, I set trigger from chan 1 and trigger level. I never try to acquire signal from chan 1, but trigger from chan 2. Of cource, I can try see if it works.

Liming

[DianeS]

Ok...but if you look at your VI, you do set up channel 1 as an acquisition.  I think that you're doing that so you can detect the scope trigger -- is that right?  I don't entirely understand what you said in your reply.

You don't have to set up channel 1 as an acquisition.  Just set it up as the trigger source, and set "trigger level" to whatever voltage you're using now to detect the scope trigger.  So, if right now the scope trigger level is set to, say, 100mV, and you're using channel 1 to detect that voltage level, you can still do that...you will set that channel as the trigger, and set the trigger level to 100mV.

Did you look at the example that I mentioned?  I do think it will help you.

d

[detech]

Yes. I looked at the examples.

Let me describe my problem again: I use this PCI-6251 card for acquiring signals from two input channles, say chan1 and chan2. I used "reference analoge edge trigger", I have to since I need some data before triggering. I use chan1 as my trigger source.  But, the error is this card can not take data from two channels with "reference analoge edge trigger". looks like, this is the limilation from PCI-6251.

From what you said, if I use saperate chan (say chan3) as trigger source, it should be OK to acquire signal from Chan1 and Chan2. I am not sure this is what you mean?

Thanks

Liming

[DianeS]

Yes...that is my first reply meant.

So you do need channel 1 as an acquisition.  I must have misunderstood your previous post.  Try using channel 3 (or one of the other available AI channels) as your trigger.

[John_P1]

Hi everybody,

This is a tricky one so I just wanted to post so there isn't any confusion here.  What you have described would not be possible on any of our Multiplexed boards.  It is a bit difficult to find, but page 11-3 of the M Series User Manual actually mentions the restriction:

When you use an analog reference or pause trigger, and the analog channel is the source of the trigger, there can be only one channel in the channel list.

We can explain the restriction by looking at the triggering circuitry (page 11-2 of M Series User Manual):

When using a reference trigger, the hardware is actively sampling data the entire time in order to create a pre-trigger buffer (i.e. the data is being MUX'ed into the ADC).  The routing of the ai channels to the triggering circuitry is after the multiplexer.  So, we have the multiplexed version of the ai lines going into the triggering circuitry MUX.  We do not allow the user to configure an AI line to be used for a reference trigger in this case since there is no way to isolate the single AI line to be fed into the triggering circuitry (e.g. we might trigger on a transition between ai channels even though both lines are DC signals).

The best workaround use the APFI lines as your trigger source.  Looking at the above image, they do not have the same restrictions as the AI input lines when used for Analog triggering since they may be directly rotued to the triggering circuitry.

If you have any questions about this don't hesitate to post back!

Best Regards,

[DianeS]

Thanks for the clarification!  I'm always happy to learn, and sorry to have led this poster astray.    I'm grateful to you for stepping in and correcting me.  Thanks again!

[detech]

Thanks for the help. I will try what you suggested. I am wondering if I connect my input signal from AI line to APFI, and use APFI as input channel for signal acquiring and triggering? If so, that makes it easier. Thanks, Liming

[John_P1]

You acquire the signal on the ai lines, APFI does not route to the ADC so you can't actually acquire data off of that terminal.  I would just jump a wire from the AI channel that you want to trigger off of to either of the APFI lines and use the APFI line as your trigger.

Best Regards,