How Do I Create a Single Delayed Output Pulse (Analog Voltage) From An Input Reference Trigger (Analog Voltage)?

[Morgen]

Hi I am working with a combustion chamber and using a DAQ system (with NI SCB-68 hardware) to read the pressure within the cylinder (as analog voltage). I am trying to output a delayed, 1-millisecond, pulse at 5 volts once the pressure within the cylinder has risen above 5 bar (which corresponds to an analog voltage of 0.25 V). I also want to record 30 ms of samples before the trigger and 220 ms of samples after the trigger. The following image visually shows what I'm talking about.

I have created a LabVIEW VI (which is attached) but I keep running into 2 problems:

1. When I run it with Finite Samples after a period of time I get Error-200281which I don't entirely understand.
2. Using the "Elapsed Time" Express VI to keep time for the pulse I cannot get a resolution of 1 millisecond, so the pulse is not generated when I set the window between 0.003 and 0.004 seconds for the pulse to be high (i.e. the resolution of "Elapsed Time" seems to be too coarse).

I am a beginner at LabVIEW so sorry if my questions are trivial or if my VI makes no sense but I have been stuck on this for more than a week. Any help would be greatly appreciated!

Thank you,

Morgen

[User002]

Here's an explanation of the error code -200281:

http://digital.ni.com/public.nsf/allkb/3E9EBBEEFA2505388625713D006AEA4D

As far as getting a 1ms pulse inside that While Loop, I'm betting that is takes mroe than 1ms to iterate. The Elapsed Time VI can handle a 1ms increment, but you've got a lot going on in the While Loop, the Write to Measurement File VI is opening, writing, and closing the file each iteration.

[Morgen]

Thank you so much for your help! The explanation of the error code is extremely helpful and that makes sense why I am unable to get the 1ms pulse. Do you have any suggestions on how I mght be able to produce the pulse without using such a complicated While Loop? I really appreciate your help.

Thank you,

Morgen

[User002]

What device are you using for your AO?

[Morgen]

I'm using an NI PCI-6259 DAQ card with an SCB-68 interface block. Once again, thank you so much for your help. I really appreciate your input.

Mrogen

[User002 · Accepted solution]

This is not a good way to trigger a pulse.
Use a DAQmx trigger to send the pulse when your acquired signal rises above 250mV like you specified.

See this for DAQmx Triggering:

http://www.ni.com/white-paper/2835/en/#toc4

[Morgen]

Thank you so much for the link! This is extremely helpful. I knew my VI was really inefficient but I wasn't sure how to clean it up. Also you were right before about the loop taking  too long to iterate. Even after I moved the "Write to Measurement" outside the loop it was still taking more than 17 ms to iterate.

Thank you,

Morgen