Which USB-based DAQ supports External triggering?

[Bagpiper]

Hi there,

We've been using the USB-6008 and later learned we could not trigger externally through the PFI0 digital

input.

Our application requires acquiring 1 sample every 20 ms. Since every sample needs to be in sync

with the process (a pulse every 20 ms), we need to trigger the data acquisition board externally.

What are our options for using a USB-based daq device? It must be able to be triggered externally,

not by software.

Thanks in advance!

Martin

[AlanArmstead]

Just to clarify, the USB-600x devices do have the capability to perform analog input operations with an external digital start trigger.  However, it looks like for your application, you need to have the ability to acquire data with an external scan clock.  A good USB option for this might be the DAQPad-6015 or DAQPad-6016.

-Alan A.

[Bagpiper]

Thank you Alan,

The issue is that for the 6008 -under the described scenario- can't handle an external clock/trigger at 50 Hz, something that seems to
be a pretty slow requirement. Could it be that the libraries (NI-DAQmx v.8.0) are not configured properly? We do get errors when trying
to use the external trigger.

Martin

[AlanArmstead]

Hello Martin,
The attached VI demonstrates 1) How to do external triggering with a USB-600x and 2) how you can test to see how fast you can go with an external trigger.  The problem with the slow rate is a hardware limitation and has nothing to do with the driver (NI-DAQmx).  You simply need a device capable of external clocking. 

The limitation with the USB-600x devices is that in a RE-triggered type of application, each time a trigger is received, the task must be stopped and re-started.  Those VIs take time to execute.  So the frequency you will be able to get will be software dependent (how fast your system can execute those VIs).  Using this method of starting and stopping the task, I was able to achieve a max frequency of about 25Hz on my machine (P4 3GHz, 1G RAM). 

As I mentioned before, you really need a device that supports external clocking.  The DAQPads I mentioned in my previous post could handle this situation with ease.

-Alan A.

Message Edited by Alan [DE] on 02-10-2006 11:15 AM