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Signal Express 2011 - N samples, 1 sample to read

Hi,

 

I have a problem setting up a simple project in Signal Express 2011 SP1.

The project requires a relay to be driven regularly (say 'on' for 1 second, 'off' for 4 seconds, and repeat). Once the relay is switched off a single set of analogue readings should be taken. My DAQ hardware is a USB6251.

I can drive the relay from one of the counters, and trigger from the falling edge of this for the analogue acquisitions.

 

I'm struggling to get a suitable timing setting for the acquisition of the analogue channels. In the help box for acquisition mode is the following tip;

 

Tip   When using LabVIEW SignalExpress in run continuously mode to acquire one sample per iteration of the task, or generate one pulse per iteration of the task, in DAQ Assistant, configure the timing settings for N samples or N pulses with N set to 1. More information.

 

Following the 'more information' link yields;

Acquiring Analog Input

 

If LabVIEW SignalExpress is configured to run continuously, and the DAQ Assistant timing settings are set to acquire continuous samples, then the DAQ Assistant will constantly acquire samples, and LabVIEW SignalExpress will read contiguous data back from the DAQ Assistant. If the DAQ Assistant is configured for one sample, DAQ Assistant only acquires one sample for the entire run of the task. If the DAQ Assistant is set to N samples, with N set to 1, then the DAQ Assistant will acquire one sample per iteration of the task.

 

All this leads me to think N samples, 1 sample to read, 5Hz sample rate would be appropriate settings.

 

However when I try this the analogue acquisition generates an error like this;

Error -200077 occurred at DAQ Assistant

 

Possible Reason(s):

Requested value is not a supported value for this property. The property value may be invalid because it conflicts with another property.

 

Property: SampQuant.SampPerChan
Requested Value: 1
Valid Values Begin with: 2
Valid Values End with: 8589934590

 

The error message indicates that I need to read at least 2 samples, 1 is not allowed, although the help files seem to specifically suggest this.

I don't want 2 readings for every time I wish to take one set of sample data. So what is the best way to set this up?

Regards,

Bandit

 

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Dear Bandit,

 

Thank you very much for your post.

 

For what you are trying to achieve I think that LabVIEW would be a more suitable platform for your application instead using SignalExpress. LabVIEW enables you to configure timing much more effectively. Is there a reason why you are not using LabVIEW instead?

 

 

Kind regards,

 

George

George Alex
Applications Engineer
National Instruments UK&Ireland
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Hi GeorgeAlex, thankyou for your reply.

Because we are expanding upon work previously done with Signal Express we need to explore the option of expanding that work. For 'simple' tasks there is a reluctance to move from Signal Express to LabVIEW. The difficulty is often defining the boundry of 'simple'. However in this instance I think we now have our answer. 🙂

Best regards,

Bandit.

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However, this doesn't explain the anomaly between what is suggested in the help file and the error generated when you try to implement it - something to look at?

Regards,

 

Bandit

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Dear Bandit,

 

Thank you very much for you post. From what I understand you wish to implement hardware timing with the use of the relay. After investigating further I found a knowledge base article that will help you understand the behaviour you are getting from your system.

 

In hardware timed single point sample mode, samples are acquired or generated continuously using hardware timing without a buffer. In order to acquire/update samples continuously and process them on a point-by point basis you need to a use a fast deterministic bus. Devices that can support this timing mode are only PCI/PXI/PCIe/PXIe. You will also find in the end of the article that USB (62xx) devices do not support hardware-timed single point sample mode.

 

If this is what you are trying to achieve, I would suggest considering buying another Input device or migrating your current application on LabVIEW. Have you had previous experience with LabVIEW? There are allot of resources online that can help you to get started.

 

Kind regards,

 

George

George Alex
Applications Engineer
National Instruments UK&Ireland
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Hi GeorgeAlex,

The relay is used to switch a sampling system on and off, I will drive this from a counter output to give a stable repeatable sample time.

The measurements will then be taken when the sampling is complete, so I will trigger this from a falling edge of the relay drive signal from the counter, with a short delay added to allow the sample to settle.

Yes, I'm comfortable working in LabVIEW, and do so when project work demands. I've now migrated to prototyping some code in LabVIEW. The possibility of expanding the basic Signal Express solution was a project management request, which I have now explained isn't a sensible route to a solution.  🙂

Thank you for your continued attention.

Regards,

Bandit

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