Scanning rate with a ISA board

I have a problem with a ISA board. I use low level I/O functions for loop scanning a input port.
I have something like that:
do{
x=inp(adress);
}while(flag);
In this way I have a good scanning rate (1-10 usec),
but I haven't any control of my program.
If I insert in loop a function like GetUserEvent() or
ProcessUserEvent() to catch a keystroke for terminating
the loop, the scanning rate becomes very low (30-50 msec).
Is there a way to control a loop like this and have in the same
time a good scanning rate?

The only other feasible way to process events during the scanning loop you have is to make your program multithreaded. With the user interface running in the main thread and this processing loop running in a second thread. After you have set it up in this fashion you can then use global flag variables to communicate process information back and forth, and use things like thread safe queues to reliably pass data between threads.
Take a look at the "Multithreading Overview" linked to in the online help in CVI 5.5. It can also be found as a .pdf file on your hard drive in the cvi\bin directory. Multithreading is a new feature in CVI 5.5 and is not available in earlier versions.
Hope that helps you out.
Jason Foster
Applications Engineer
National Instruments
www.ni.com/ask

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DAQ if scan rate with external timebase, what will happpen to channel rate

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When an external timebase changed by using the RTSI bus as described in http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/5601c8268d625a3c86256cf50071199d?OpenDocument, this will change the DAQ cards Master Timebase. The MasterTimebase signal, or Onboard Clock, is the timebase from which all other internally generated clocks and timebases on the board are derived. Therefore, both the scan and channel rate be based on the external timebase.

How to set scan rate with NI Switch scan voltages

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Hi Hema,
CJC is an acronym for Cold-Junction Compensation, and this value adjusts for the change in voltage caused by the thermocouple wire to copper wire junction.
For example, a J-type thermocouple will have thermocouple wire consisting of iron and constantan metals. When these iron and constantan metals meet the copper at the switch connection, a difference in voltage results. This difference in voltage is the "cold-junction". The difference in voltage resulting from the iron and constantan connection in the thermocouple is the "hot-junction". When you measure temperature using a thermocouple, what you desire is the "hot-junction" change in voltage. Unfortunately, the DMM is going to measure the sum of both the "cold" and "hot" junctions, and a CJC measurement is needed so we can adjust the measurement to remove the undesired offset.
Once Cold-Junction Compensation is performed, converting from voltage to temperature is fairly simple. Each thermocouple type has its own temperature to voltage conversion equation and associated coefficients. Here's a great resource for the equations, coefficients, and specific voltage to temperature tables:
NIST ITS-90 Thermocouple Database
http://srdata.nist.gov/its90/main/
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Chad Erickson
Switch Product Support Engineer
NI - USA

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VKral,
Here are a few KnowledgeBase entries that I hope will clear up the confusion:
Data Acquisition Sampling Terminology
http://digital.ni.com/public.nsf/websearch/DBE7AC32661BCF9B86256AC000682154?OpenDocument
How Is the Actual Scan Rate Determined When I Specify the Scan Rate for My Data Acquisition?
http://digital.ni.com/public.nsf/websearch/5782F1B396474BAF86256A1D00572D6E?OpenDocument
Hope this helps. Have a great day!

How can I record several channels of data at different rates with the same card?

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Hello;
At this time, the NI DAQ boards have only one scan clock, so, you can only define one valid scan rate for all of your channels.
The better approach for you is to sample all channels at the highest rate you will need, and then discard the samples you don't need to save.
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Using this DAQ example, it may be easiest to simply place a wait function in the loop and software time when the data is extracted from the board. You can also take a look a the NI-435x pallette and the examples it ships with, along with the timing specs on page 3-3 of the 435x manual and set the filters to meet your needs:
http://www.ni.com/pdf/manuals/321566c.pdf
Regards,
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When an external timebase changed by using the RTSI bus as described in http://digital.ni.com/public.nsf/3efedde4322fef19862567740067f3cc/5601c8268d625a3c86256cf50071199d?OpenDocument, this will change the DAQ cards Master Timebase. The MasterTimebase signal, or Onboard Clock, is the timebase from which all other internally generated clocks and timebases on the board are derived. Therefore, both the scan and channel rate be based on the external timebase.

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THANK YOU
FOR YOUR HELP, JOE BOB CRAIN

I think the best way is to generate an array of time values to send to your spreadsheet BEFORE channel 0 data.
To generate the array you can use Ramp Pattern.vi from 0 to 'Actual Scan Rate' (from AI Start) * number of scans performed.
That way in your spreadsheet you will have the first line same as now, next the columns with time(secs), channel 0, channel 1... channel 7.
If you need the time column with date and time string, you can use Get Date/Time in Seconds.vi when you begin acquisition, then sum the seconds obtained to the array calculated as above and next use Format Date/Time String.vi to obtain the time stamp you need, next build array of time stamps and sent to your spreadsheet.
Hope this helps
Roberto
Proud to use LW/CVI from 3.1 on.
My contributions to the Developer Zone Community
If I have helped you, why not giving me a kudos?

How to calculate min/max scan rate of PCI-MIO-16XE-50

My board: PCI-MIO-16XE-50 , 20MHz timebase
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Hi May
Check out the knowledge base locate at http://digital.ni.com/public.nsf/websearch/EA4E306828AF05B586256B8F00661A8B?OpenDocument . I think it answers your question.
Serges Lemo
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Calculating motion rate for given line scan rate

Dear friends,
I need to understand the calculation of motion rate for given line scan rate.
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Attachments:
Chain_gear.jpg ‏17 KB

It sounds like you are mixing two methods of using a line scan camera.
The first method is to run the camera at a fixed speed. If the speed of your belt is constant, this can work okay. To calculate the desired speed, let B=belt speed (mm/sec), L=line rate (lines/sec), R=mm/line (0.1mm/line in your case)
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Bruce
Bruce Ammons
Ammons Engineering

Converting Units in LabView and specifying sampling rate with Universal Library functions

Hi,
I am having trouble converting units in LabView 7.0 and having it write to a file / output on a chart in Nm instead of in volts. I can't seem to find any straightforward instructions, eventhough it seems like a simple task.
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If anyone could help me out, I would greatly appreciate it - I have tried other sources without much luck!
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Jenna

Are you really needing to change units or do you need to scale the voltages read from your sensors? If it's scaling, it would be trivial to set this up in mAX or with the DAQ Assistant if you were using an NI board. Since you are not, then you could use the Scaling and Mapping Express VI. The y axis of a chart is just a label. You can use the text tool to change it to anything you want.
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Keithley 2701 LV driver: Setting the scan rate?

I downloaded the Keithley 2701 LV driver and the example programs work great. There is just one question that I have. I cannot find anywhere in the examples or in the LV drivers how to change the scan rate or scan interval. Everything else I'm able to change, such as number of points, channels, voltage levels, etc., but I can't figure out how to change my scan rate to once every 0.5 seconds or 1 second.
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I'm using LV8.2
Jeff

I am using the Keithley 2700 with the 7700 cards. There is a vi named "ke27xx Configure Aperture Timing.vi" in the "ke27xx.llb" .
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LV 8.2
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During continuous scanning thermocouples with a SCXI 1120 module is the cjc reference read each scan?

Did the developers of the DAQmx drivers improve the temperature scan capability of the SXCI 1120 module over the methods used in the traditional drivers?
When using the SCXI 1120 module for temperature measurement and in the continuous scan mode with traditional drivers the cold junction reference was only checked on the initial start of the scan. In order to maintain accuracy it was necessary to restart the scan.
For the most part I have switched to the 1125 module for thermocouple measurement but I need to use some 1120 modules on a test stand now and I will be using the DAQmx drivers on these. I was in hopes that with the DAQmx drivers this behavior was corrected.
I can conduct testing to determine this but would like to know ahead of time if this is still an issue.
Thanks

Thank you Ben for the answer.
I checked the article you referenced appears to me that I will be adding the physical address in the channel strings then do the voltage to temperature conversion and then correct the appropriate channels associated with the cjc in question. I will also need to adjust some on board jumpers for the SCXI 1120. If I am wrong please correct me.
If scanning the physical channel is possible as suggested in this article, would it not be possible to create the channel as a virtual global channel and reference it as the cold junction in Max? Then the temperature adjustment could be done at this level of acquisition rather than having to sort this out in the LabVIEW program. This takes me back to the days with LabVIEW 4.0 before we had the virtual channels and when we had to handle the temperature conversions and other scaling ourselves. Again if I am incorrect please let me know, for I would like to use the method recommended in the article if I don't have to perform all the calculations in the application.
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Non-time based scan rate

How do I acquire analog data with a scan rate based on a digital trigger and not based on time. I have two signals one analog and one digital. I want to acquire an analog data point every time the digital signal goes from high to low and from low to high.
Thank you.

I don't think LabVIEW DAQ hardware supports triggering on both edges. It appears you have to trigger on one or the other.
The only thing that I can think of to try is to take your analog signal and split it into two different analog inputs. Use the example "Acq&Graph Voltage-Int Clk-Dig Start.vi". Insert it in a new vi twice. You will have to open it and modify the connector so the Physical Channel and Trigger Edge are inputs and the graph is the output. Then wire each one with a different analog input and trigger edge and run them sequentially. Hopefully it will run fast enough for you.
Randall Pursley

During slow scan rates, are events captured between scans?

We are using 32 thermocouples to measure a heated cabinet performance and are using the Continuous Acquisition to Spreadsheet VI. I am saving the data to a text file while plotting the data to a chart in Labview 6.1. I only wanted to read temperature every 60 seconds (0.01667 scans/sec) but set the scan rate rounds off to 2 decimal places, so at 0.02 scan rate we read data every 50 seconds. I set the buffer size to 1 and the number of scans to write at a time to 1. Does this mean I have a lag of one scan in the buffer or am I seeing the latest scan written to the chart? Also, temperature events between writes to the screen sometimes show up on the chart, sometimes not. It seems to depend on how long the e
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Willard,
You can actually change the digits of precision in LabVIEW to display more than 2 decimal places. If you right-click on the scan rate control and select 'Format and Precision' you can alter the digits of precision to include 5 decimal places.
By setting the buffer size to 1 you are setting a memory size to hold only 1 scan at a time. By setting the number of scans to write at a time to 1 that means that every time the AI Read VI is called within the while loop, the VI will only read 1 scan from the buffer. Therefore, the hardware is set-up to read 1 scan every 50-60 seconds and so the hardware places that scan in the buffer. Then every time the AI Read VI is called in software, it reads 1 scan from the buffer. So, if the AI Read VI already read
the 1 scan from the buffer, it will wait until the buffer gets the next scan from hardware (50-60 seconds) and then plot it on the chart. So, with the set-up you have, you will not see any lag of temperature data. Once the hardware scans, the software will almost immediately read that scan. I hope this helps.
Regards,
Todd D.
Applications Engineer
National Instruments

Scanning rate with a ISA board

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