Agilent 3458A DMM waveform capture problem

[hammer3]

I’m trying to use the 3458A to do a real-time capture of the data comprising a periodic ramp that goes from 0 to 4v. I want to do this in REAL TIME capturing all data on the FIRST ramp after the trigger.  I have a function generator connected to the 3458 delivering this waveform. I want to capture and store the voltage values into an array for post processing. I’m using the Application Read Waveform.vi that comes with the HP3458A driver. Using the default “number of samples” and “sampling rate” I get the expected data display (see goodwaveform.jpg) . 100 samples at the sampling rate of 50Hz (50 samples/sec) captures 2 seconds of the waveform. The waveform is a 500mS ramp with 50% duty cycle. This is good.  However I want to gather data ONLY on a 500mS segment of the waveform, and it seems logical that I should be able to set the sampling rate to 1000 samples/sec (1ms per sample) and read 500 samples to view a 500ms section of the waveform. Obviously I’m wrong because the second screenshot shows the results of those settings. It appears the sampling rate is closer to ~72 samples/sec instead of the desired 1000 samples/sec. What am I doing wrong? VI attached.

[crossrulz]

I haven't looked up the specs yet, but many DMMs will only sample at a max rate at a given resolution.  You might have to lower your resolution in order to get your higher digitizing rate.

[hammer3]

I'm already at the lowest resolution.

[_Tim]

Have you taken a look at the set of data that's being read in any way other than the graph? It may be useful to verify the actual data coming in and make sure that it's not just an issue with the way the information is being displayed.

[hammer3]

The graph displays the data exactly as it would be displayed in an Excel sheet. I've done that before. I found the vi I was using did NOT put the 3458A in FAST mode as I thought it did, so I enabled FAST mode, and now it behaves correctly except I can't find a way to use LEVEL TRIGGER to get the capture to begin consistently at the same ramp voltage level.

[_Tim]

What is preventing you from using Level Trigger. Is it just not giving you enough accuracy, or is it causing other issues with your code?

[hammer3]

I'm using it but it has no effect. There is no consistent triggering. It triggers randomly, like autotrigger. Could be I have the wrong SEQUENCE of commands, or something prior to or after the trigger command is missing. I don't understand the DMM or its language well enough to know what to do.

[_Tim]

If there are examples included with the drivers, it may be worth looking for some code that utilizes the triggering function to see how it's implemented. It might also be worth looking up the specs of the device to check on triggering level and sensitivity.

[hammer3]

I've done all that already, prior to posting the problem.