Triggering in current measurement on NI 9208

[Optifox]

I have an NI-cDAQ 9188 CompactDAQ measurement chassis, containing an NI 9205 voltage ADC and an NI 9208 current ADC. Both are working fine in continuous mode, but I'm having trouble with triggering for finite measurements.

I can set up a trigger on the 9205 voltage measurement module just fine - I just use a DAQmx Start Trigger (Analog Edge) node, and make sure that the channel on which I'm triggering is the first one in the list of channels for that task. (I'm building tasks programatically rather than creating them in MAX). This works fine. However, when I try to do exactly the same thing with the 9208 current measurement module, I get the following error:

Error -200265 occurred at ADC monitor.lvclass:Read data.vi

Possible reason(s):

An attempt has been made to use an invalid analog trigger source.

Ensure that the trigger source you specify matches the name of the virtual channel in the task or matches the name of a non-scannable terminal that the device can use as an analog trigger source.

Property: Start.AnlgEdge.Src
Corresponding Value: Current

Valid Choices: none

Device: OCM-DAQ

Task Name: _unnamedTask<9C>

"Current" is the name of the virtual channel I've used to build the task. It's the only channel in the task.

When I got this error on the voltage module, it said "Valid Choices: Voltage1", which is how I found out that the triggering channel needs to be the first one. For current, though, it's not giving me any valid choices.

Does this mean that I can't get an analog trigger using current on the NI 9208? Can I access the same trigger as the NI 9205 uses (being the voltage signal on the NI 9205)? I've tried putting some terminal choices such as /Dev1/StartTrigger in as the trigger source, but I get the same message.

I can probably build the current and voltage channels into the same task, and use the voltage channel as the trigger, but then I would have to use the same timing characteristics for the two channels, and I want to read the voltage at a higher rate than the NI 9208 can handle.

It looks like I just can't make a current channel the trigger source, then. Is that the case?

[JVP · Accepted solution]

The trigger channels are Voltage triggers, and cannot be used as a Current trigger.

Regards,

Jig P

[John_P1]

Hi Optifox,

To clarify Jignesh's post, it's true that the 9208 does not support analog triggering.  The only current C Series modules to do so are the 9205 and 9206.

However, you may trigger the 9208 off of the analog trigger from the 9205 if desired.  The analog trigger circuitry is just an on-board comparitor that outputs a digital signal called the Analog Comparison Event.  You may use this Analog Comparison Event as a digital trigger for your 9208.  In order to select the Analog Comparison Event for your digital trigger source, you need to right-click on your DAQmx Terminal control/constant, and select IO Name Filtering... to enable advanced terminals:

       

Alternatively, if you want to run the acquisitions at the same rate*, you should be able to just put the channels from the two devices in the same task.  You can do this by calling DAQmx Create Channel twice as part of the same task (Mod4 is my 9205, Mod2 is my 9208):

* Technically you can run the task faster, but the 9208 will repeat samples if you set a rate larger than its maximum sample rate of 500 Hz.

Best Regards,

[Optifox]

Thank you both for your help! It's good to know that the analog current signal simnply can't be used to generate a trigger.

I ended up building the voltage and current channels into a single task, and using one of the voltage channels as the trigger. I'm running the voltage sampling at 20 kHz, so I get about 40 identical current values in a row, but I can simply interpolate to get a smooth current readout that matches the voltage data (I'm inputting a function generator running at a whopping 2 Hz, so nothing changes very quickly).

That said, the idea of using the 9205's trigger to trigger the 9208, while being in separate tasks, is very interesting, and may become valuable later. The Advanced Terminals setting seems to be just what I need for this. Thanks!