Digital read pulls line low

[Butch]

I am using a DAQPad 6020E USB for acquisition and control.  One of the digital lines is configure as an input and connected to a circuit that outputs a 5V signal until a threshold is reached, when it goes low.  When I start the software the line is pulled low, which triggers an alarm.  I don't understand why a line that is configured as an input would do this.  Why does the DAQPad cause the line to be pulled low?  I believe this only occurs the first time through the loop.  How can I prevent this behavior?

[Jesse_O]

Hello Butch,

I have setup a 6020E, and I have connected a digital line to a high impedance scope (1M).  On the scope I read in 5V since the 6020 has a pull up resistor.  When I start to acquire data on that channel I do not see the channel drop low.  I then set the scope to trigger off a falling edge to see if the line drops for a small period of time, however I cannot reproduce the behavior you are seeing.

Can you reproduce the behavior running a shipping example?  Is it possible that your external signal is falling low?  If you disconnect your external circuit and scope the line can you see the line drop low?

Regards,

Jesse O.
Applications Engineering
National Instruments

[Butch]

Jesse,

We watched the digital line with a scope and it is definitely going low.  I ran the vi with the light bulb turned on.  The digital line goes low when another vi runs.  This one is an analog input and analog output that ramps up to a set voltage and closes.  It seems as if the configuration of the analog channels is changing or the configuration of the digital line.  All the analog inputs are single ended.  Any ideas?

[Jesse_O]

Hello,

Previously I tested the digital line while creating another digital operation.  This did not cause the line to ever drop low.  I just tested a digital input while creating an analog input task and I do not see the digital line drop low. 

If this is something you can consistently see, can you put together a small piece of code (perhaps just two shipping examples) that reproduce this behavior?

Regards,

Jesse O.

[Butch]

Jesse,

I have attached an example that shows the behavior.  Each time it runs you may or may not get the same results.  It uses named channels set up in Max.  "10kv Enable" is configured as a Write to Digital Line, "Read" to Read a Digital Line.  It seems that using these "Easy DIO" vi's results in all of the digital lines being configured every time one of them runs.  Is this usual behavior for this combination of vi's and the DAQPad 6020E?  The manual says the channels can be configured individually, but it doesn't seem to work that way.

Butch

[Butch]

Sorry, I forgot the attachment.  Here it is .

[Dennis_Knutson]

Do you realize that each time you run the Write to Digital Line or Read From Digital Line, it calls the DIO Port Config? Do you also realize that because you have no dataflow between the functions, you have no idea of the order they may execute in? Try putting a single DIO Port Config at the beginning and configure the inputs and outputs just once. Then, use the DIO Port Write and DIO Port Read. Wire up the error in/error out clusters to control the order of execution. Open the diagrams of Write to Digital Line and Read From Digital Line to get an idea on how to use the the lower level functions.

[Butch]

DIO Port Config configures only one named channel, so a single instance is not enough.

I did as you suggested, with the addition of a second DIO Port Config, and the digital input line is still pulled low when the output line is configured.  It goes back high when the second DIO Port Config runs.

While observing DIO Port Read and Write during the execution I noticed that they both generated Warning 10612: The specified line does not support transfer in the specified direction.

Is this behavior peculiar to the DAQPad 6020E?

[Dennis_Knutson]

Well, it has been a long time since I've used traditional DAQ functions but I recall being able to configure each line in a port by specifying multiple channels (i.e. 0,1) and using the line direction map to set them as either input or output (i.e. 01 or 01).