using PFI 0 line on DAQ 6008 to trigger analog acquisition

[glin2]

Update: Sorry, I accidentally posted this message twice due to poor internet connectivity. Please ignore the other message in the thread that is similar to this one! 

Hello,

I am attempting to use the PFI 0 line on the DAQ USB-6008 to trigger an AI and an AO channel at a user-specified frequency. The 6008 is not listed as a device that can use the examples I've found so far, but the specification manual states:

"When an analog input task is defined, you can configure PFI 0 as a digital trigger input. When the digital trigger is enabled, the AI task waits for a rising or falling edge on PFI 0 before starting the acquisition. To use AI Start Trigger (ai/StartTrigger) with a digital source, specify PFI 0 as the source and select a rising or falling edge."

However it also implies that an external digital signal is required for the PFI line. Can I trigger on the 6008 by itself, or do I need an external digital signal (like a pulse train)?

I'm still pretty new to LabView, so any advice is appreciated.

Thanks,

Gigi

[Shane-C]

Hello Gigi,

The PFI0 is indeed configurable as a digital trigger, but it cannot be used as an analog trigger.  As the manual states, the digital trigger can be used to trigger an analog input - meaning, when setup as a digital trigger, the analog input task will wait until there it a digital input before the analog input task starts.  So yes, to use this method you will need an external digital line to go high, or low in order for your analog input task to begin.

It sounds to me like you want analog triggering, which can't be done with the NI 6008.  

See this link for more info about triggering:

http://www.ni.com/white-paper/4329/en/

If you want to know which devices support analog triggering, take a look at this page:

http://digital.ni.com/public.nsf/allkb/E761255A5729CD2E862570310075A23A

[glin2]

Hello Shane,

I believe that configuring a PFI0 as a digital trigger for an analog input may work for my application. Would I wire my differential transducer to two analog input lines on the 6008 and then specify digital triggering by PFI0 in the DAQmx Assistant? Or do I need to wire PFI0 to one of these AI lines and/or is there a better way to do this?

Basically: I am trying to simultaneously trigger image acquisition from a camera and from a pressure transducer at a user-specified frequency. In theory, the USB 6008 would send out an analog trigger to the camera pins to "snap" an image at some frequency. Simultaneously and at the same frequency, it would trigger an analog input from the transducer. In the past, I've been successful at using the USB 6008 to send out an analog "trigger" (i.e. I used DAQmx VIs to pull up the voltage output of an AO channel from 0 to 5V) to the camera. Now I would like to synchronize the image acquisition event with analog input from the transducer.  

Thanks again for the help!

Gigi

[Dennis_Knutson]

It sounds to me that you should be using a digital output as the trigger source. Try wiring it to both the camera and the pf line.

[glin2]

Hi Dennis,

Once I've done the wiring, would I then select PF10 as a digital trigger source for the DO line, and then that DO line as the digital trigger source for my AI line? 

Thanks,

Gigi

[Dennis_Knutson]

You would specify the trigger for your analog acquisition. You would write a digital pulse at the rate you want. Note that the digital outputs are software timed and are subject to considerable jitter. You can't achieve much more than 1000 samples/sec and since you need 2 samples per pulse, your trigger would occur at a rate no higher than 500 Hz.

The trigger generation would be in one loop and the analog acquisition in another.

[Dennis_Knutson]

To clarify, you specify the pf line as the trigger source.