continuous daq w/digital PRE trigger

[Greg Cole]

Hi All,

I'm working on a data acquisition app where I use an external digital
trigger. I've loaded up the example VI that lets you do continuous
DAQ with a digital trigger and have it working fine. Now what I'd
like to do is incorporate 1000 pretrigger scans into this VI. I've
worked on several permutations without success so I am posting here to
see if anyone else has accomplished this.

I could acquire my data with a straight digital petrigger approach but
the continuous buffered DAQ has features I'd like to exploit.

Thanks in advance for any info.

Greg
gcc9@cdc.gov

[Russell]

Hello Greg,

This is actually a VERY good question and I'm glad you posted this on the Developer Exchange for all to see. Currently, with the E Series boards you cannot perform a Pre-Triggered acquisition that runs continuously. (So you have discovered)

The workaround is to use the software form of triggering known as "Conditional Retrieval." For the most part, this NI-DAQ feature allows those with DAQ boards that do not support analog triggering to check for these analog conditions in software. The boards begin acquiring data and only return data from the buffer once the trigger conditions have been met. So it really basically serves the same purpose.

The point is, when using Conditional Retrieval, you are not faced with this "Continuous Pre-Trigge
red" limitation that you have noticed.

There is a LabVIEW shipping example that demonstrates this called "Cont Acq&Graph ExtScanClk Soft A-Trig.vi" Though it is technically an analog trigger example, you should still be able to use it for your application.

I hope this helps!

Russell

Applications Engineer
National Instruments
http://www.ni.com/support