Problems with "Most Recent Sample" value for "RelativeTo" DAQmx Read Property using CompactDAQ

[cpschnuffel]

Hi there,

I get in trouble when I try to aquire the next N samples from an already started continuous scan using a CompactDAQ system.

My hardware: cDAQ-9178 with modules 9474, 9472, 9411, 9221, 9201 and 9263 (inserted in that order starting with slot 1)

One of my tasks: Acquiring voltages with the 9201 and 9221 and time-correlated angular encoder values with the 9411, so for each analog sample I need the matching encoder value. I did that several times in the past using M-series devices (PCI), and that is not the actual problem.

My challenge: I need absolute angle values during the whole test process and know the initial angle only when I start that process, so I have to use one long scan (containing two tasks, one for the AI and one for the encoder), and from this scan I have to take samples and sub-scans several times during the process. So I need a functionality like "Please give me the next 1000 samples from now on" (Some milliseconds more or less doesn't care, I need no hardware trigger)

In the past I realized this with PCI cards and the "RelativeTo" and "Offset" DAQmx Read Properties, setting "RelativeTo" to "Most Recent Sample", "Offset" to 0 and then using the DAQmx Read VI.

But this doesn't seem to work with CompactDAQ. If I do this (see attached example) with 1000 samples at 1000Hz sample rate, I assume a duration of about 1s for getting the samples. But the duration is much smaller (until down to 0s), so it seems that the "Most Recent Sample" is not really the most recent one, but an older one.

Please take a look at my VI attached (LabVIEW 2010SP1). What am I doing wrong?

Thanks in advance for your help!

Carsten

[cpschnuffel]

Hi again,

some additional informations:

- This problem happens only with "real" hardware; a simulated CompactDAQ system always returns about 1s for duration.

- I use LabVIEW 2010SP1 with DAQmx 9.2.3, MAX 4.7.7f0 on Windows XP SP3

cpschnuffel

[MattLawson]

Hi,

Any resolution to this issue?

I am having identical issue with LV 2014 SP1, DAQ MX 14.1, cDAQ-9174.

"Most Recent Sample" property node DAQMxread does not seem to be accepted (but no errors are reported). 

DAQmxRead is always using "First Sample" as the reference point, regardless of the setting.

Thanks.

Matt