How do I measure the time between two triggers using a scope card?

[DFWyatt]

I have a USB Scope (USB-5133).  It has two channels.  I will be applying a voltage to a device, and monitoring it's response (a steady return voltage).  When I disable my input voltage, the return voltage will increase.  I want to measure the return voltage from the device when I disable the applied voltage. 

 In other words, I want to start the timing when my input voltage (on Channel 0) drops below a certain voltage, and stop the timing when the return voltage goes above a certain voltage. 

Does anybody have an idea how to set the triggers of NI-SCOPE to do this? I have never used NI-SCOPE before.

Thanks in advance!

[JoeFriedchicken]

Hi,

The NI-Scope driver installs several examples for using your device. The one that would be best to start with is "niScope EX Measurement Library.vi" and its default installation location is "C:\Program Files\National Instruments\LabVIEW 8.2\examples\instr\niScope". You can also find that folder in the Start menu: Start » Programs » National Instruments » NI-SCOPE » Examples.

A few important distinctions to make with a scope are the trigger types and the idea of a record. Scopes have two basic kinds of triggers: start and reference. A start trigger tells the scope to start listening to samples and write them to on-board memory, and a subsequent reference trigger tells the scope which of those samples to keep and return to you. You don't have to specify a start trigger, and the device will start immediately putting samples in its on-board memory. You must configure a reference trigger, and you can choose from several types of reference triggers. In your case, you would want to configure an analog edge reference trigger for a falling edge on channel zero. The example above allows you to do this without any modification. If you want more information about triggering, refer to the driver documentation in Start » Programs » National Instruments » NI-SCOPE » Documentation » NI High Speed Digitizers Help and navigate to Fundamentals » Triggering.

A record is a collection of samples recorded around the reference trigger. When you fetch a record from the scope, you ask for X samples. The reference trigger will allow you to specify how many samples in the record are before the trigger and how many are after. This is the "Reference Position". A reference position of 0 means that all of the samples in the record were recorded after the reference trigger (the first sample being taken at the same time as the reference trigger). A reference position of 100 means that all of the samples in the record were recorded before the reference (the last sample being the taken at the same time as the reference trigger).

For your measurements, I would set an analog edge reference trigger for your first event and then fetch all of the samples after that event. That means using a reference position of 0. Finally, when setting the record length and sample rate, I would take your device's on-board memory size into account: do not try to record and keep more samples than the scope can hold.