The change of a global channel applies to all tasks that use that global channel??

[netwon]

I got following message from NI-DAQmx Help under topic "global channels":

"If you add a global channel to several tasks and modify that global channel with the DAQ Assistant, the change applies to all tasks that use that global channel."

However, after I chang a channel # connected to a thermocouple in a global channel, either with DAQ Assistant or MAX, the channel # in the tasks that use this global channel remains unchanged! I created this global channel in MAX and copied it to my tasks through DAQ Assistant,

Anybody knows why? Thanks a lot.

[Otis]

Hi netwon,

When you use a global channel all values should be updated, so long as you saved the changes that you made.

Can you get it to not save again?  Could you then tell us what steps you did (1. Open MAX 2. Create New DAQmx Global Channel, 3. etc...)?

That should help us try and see what step is messing up, or if it's an actual error we can work on a way to get around it.

Thanks for the extra info,

[netwon]

I did save the changes. Below are detailed steps I did. I am using LabVIEW 7.1.

Open MAX—Right click Data Neighborhood to create new—Select NI-DAQmx Global Channel—Analog Input—Temperature—Thermocouple—Select a physical channel—Enter name: My Temperature0--OK

Open LabVIEW, Create a new blank VI—Drag a DAQ Assistant on the Block Diagram—In the launched window, select Analog Input—Temperature—Thermocouple—Select Global Channel: My Temperature0, check Copy Global Channels box, then finish—Then OK—Save as test.vi and exit LabVIEW

Return to MAX, click My Temperature0 and change its settings: Change its Max temperature input from 100 degC to 50 degC, then SAVE channel.

Open test.vi again. Double click DAQ Assistant I just created. See what’s updated? NOTHING!! The Max input is still 100 degC!!!

Hope you can help. Thanks again.

[netwon]

Forgot to mention: I tried many times, either changing channel # or changing other settings. None of changes could be updated.

[Otis]

Hi Newton,

I see what's going on.  When you make the DAQ Assistant, and you select a Global Channel it selects "Copy Global Channel".  The means that your DAQ Assistant is referencing a copy of the Global Channel at the time it was created.  Since you are not using the Actual Global Channel it will not synchronize the two.

As an experiment you can then open the DAQ Assistant with the Original Global channel, select "Add Channel", and then add the channel that you updated in MAX (the same name as the original channel).  You will see that this new channel uses the same information that was last saved and once again makes a copy of the channel that you created in MAX.

Hope this helps clear up why you are seeing the problem.  Since you are dealing with a copy of the Global Channel the real Global Channel can be changed, but it does not affect the copy.

If you think it should be the other way around, then please make a Product Suggestion.

Let me know if you need anything else,

[netwon]

Hi, Otis,

Thanks for your helpful message. Following your advice, I tried to add a global channel. However, the choice of Add Global Channels was grayed out. I only created two global channels for thermocouples. They do not support Add Global Channels?

Thanks again.

Netwon