Open Channel

[Tebefia]

Folks, am trying to code a particular behavior using Labview, and would appreciate any help in this regard.  I have a Channel configured as AI2 (Analog Input 2) of my DAQ device, and what is on the channel is a 1D array of Doubles if there is any data signal in the Channel.  The behavior I desire to code in Labview is such that if there is no electrical signal connected to the AI2 of my DAQ device, then the Boolean Button on my Front Panel that is associated with that Channel should start blinking, just to indicate to the user that there is no data signal in that particular channel.  In other words, the Blinking should indicate "Open Circuit". 

I thought my attached code could accomplish that, but when I tried the code with nothing connected to that channel, the Boolean button on the Front Panel never blinked.

I would appreciate any suggestion.

Thanks in advance.

[natasftw]

http://digital.ni.com/public.nsf/allkb/4E9B8B3F8FD3A31B862562B5004EA1D9

Read that.  The short version:  when there isn't anything driving the voltage on the channel, you cannot expect it to have any specific value.  You're expecting it to have a value.

[Tebefia]

natasftw, thanks for your prompt response to my question, and moreso for the link.  I have read it and can also understand what you saying.  Why I initially compared whatever data signal on the channel to "0.1" was because I assume there will always be some enviromental noise.

But if I may ask again, what lines of Labview code do you think can detect an disconnected signal channel (Open) and is reflected by a blinking object that is on the Front Panel?.

Regards.

[GerdW]

Hi Tebefia,

how will you recognize a "disconnected" signal? How do you recognize a "connected" (or "valid") signal? What signal values do you expect?

When you can answer those questions you can easily create (LabVIEW) code to detect your conditions…

[Hooovahh]

People falsly believe that an analog input should work like a DMM.  When I have nothing connected to my DMM it reads 0V.  Well with an analog input it will read the voltage potential between those two points in the air.  To avoide this people usually put in a weak pull down resistor, which will help the signal become 0V when nothing is connected.

[RavensFan]

Can you clarify what a "weak pull-down resistor" is?  When I think "weak", I would think of a low resistance value.  But I think you'd actually would want a very high resistance resistance so it doesn't affect the measurement of a real signal much, but would dominate and pull the signal to zero if you had infinite resistance between the leads.

[Hooovahh]

You are right sorry. I was thinking of another circuit. A higher impedance signal should effect the signal less. Which is what you would want from an analog measurement.