Syncing Voltage Reading Acquisition Start on Multiple Instruments

[Akello]

Hello,

I'm doing a lot more work with intrument control lately so please forgive the uptick in questions.

I'm dealing with a unique issue I haven't encountered before and I could use some advice. Previously when I asked about multiple instrument control it was always one instrument running continuously while the others would have their parameters dynamically updated; for example voltage readings with an NI-DAQ with adjustable DC power supply voltage. For such instances, I was recommended looking into the Producer-Consumer Architecture. In other cases, I've used multiple channels of a single instrument with no problems.

Now I am trying to monitor voltage on two seperate devices; a Keithley 6517A Electrometer and an NI USB 6212-BNC. I'm trying to take a finite number of samples over several seconds so the start time of reading acquisition needs to be synchronized. I didn't think the queuing of the Producer-Consumer would be helpful here so I tried configuring both instruments and then splitting the error wire in two. Predictibly this caused a lag in the acquisition time of the instruments as can be seen in the attached .jpeg. I know that the way I am doing it leaves me at the mercy of the instruments' timing but ths is the first time I've had to do this kind of setup and I am having some trouble.

Would very much appreciate some advice for this and future applications.

Hope to hear from you,

Yusif Nurizade

[johnsold]

Yusif,

I do not have the Keithley VIs so i cannot tell what is going on in those.

From looking at your VI I have several quesitions.

1. The DAQ Graph appears to be connected to the electrometer output and the Electrometer Graph is connected to the DAQmx Read. Is this intentional?

2. Does this VI only take one reading (from each instrument) and then quit? I would have expected a loop around the Read VIs.

3. What trigggers the electrometer to take a reading? What causes the analog signal to change so that it triggers the DAQ?

What I have done in situations like this is to trigger both devices from a digital signal which could be generated by the external system or a digital output from the DAQ device. Another option is to run the DAQ acquisition continuously and monitor some signal which is common to both instruments on a second DAQ input channel. Then analyze the data  in software to synchronize the results.

Lynn

[Akello]

Lynn,

As ever, you are my rock.

To address the points you've brought up;

1. Thank you for pointing up this mixup with the graphs, I've adjusted in the updated VI.

2. Both instruments are set to log 50 samples/second for 10 seconds.

3. As I mentioned, I have not had to deal with synchronizing two measurement devices like this up until now so the necessary triggering is new to me. I imagine I could set both to trigger on a rising pulse, for example, but I am dealing with single pulse captures so I thought an external trigger might work better. The only External Trigger label on the back of the Keithley is a BNC input, could I connect that to a P0 channel on the DAQ for triggering? 

Since I'm new to this, I came up with a temporary fix that works by pausing the execution of one of the instruments by approximately the delay time but I want to move into effectively triggering both devices.

Thanks again,

Yusif Nurizade