Q: Why do "virtual channels" mysteriously disappear?

I am a newbie to Labview 6i and have been encountering a recurring problem.
I have been using the sample VI "Getting Started Analog Input.VI" as a
sample to practice basic techniques by making incremental changes and
observing the results. The first subVI in this block diagram is AI Config.VI
which requires an input "I/O" which tells it which channel to use.
Supposedly, I can simply enter a list such as 0,1,2,3 etc and have this
interpreted as aich0, aich1... of the specified device, but the only way I
have found to get the program to work is to create a "virtual channel" in
Measurement And Automation Explorer. If I do this, I can get then choose the
virtual channel and VI will run.
The problem that I cannot figure
out is that every now and then, all the
virtual channel information is deleted and must be recreated. I can't see
what it is I am doing that causes this to happen, maybe I'm clicking when I
should double click or something, but it sure is frustrating. I was just
running it a minute ago and they were there at the beginning of the session
and at some point just disappeared without ever acutally running a VI.
Any ideas would be greatly appreciated.
Jon Juhlin
[email protected]

Jon;
If you right click at the channel control and select the option named "allow undefined names", you will be able to type in the channel number as well as select the virtual channel.
Hope this helps.
To avoid the virtual channel to be deleted you can choose the option to save the configuration file in MAX. That will save all your channels to one file and everytime you open MAX, or try to access the virtual channels in Labview, the ones you created will be there.
Hope this helps.
Filipe

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I create a bunch of virtual channels in MAX - some with double assignment to a single AI port.
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natashw already told you that it's expected behaviour when you leave the analog input open. There are two aspects to this:
1) The analog input amplifier is a high impedance operation amplifier. It's minimal stray capacitance is high enough that it gets charged through very small leakage currents in the amplifier input stage, but the high input impedance doesn't allow those capacitance to discharge quick enough to stay at a defined voltage. So your input is usually very slowly floating to one of the power supply rails depending what transistor side has a slightly higher leakage.
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Q: Why do "virtual channels" mysteriously disappear?

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