Unwanted 5VDC (uS) Pulses on DAQCARD-1200 Digital Outputs

[Doug1974]

I'm currently using a DAQCard-1200 PCMCIA multifunction DAQ card to automate some "push-button" features on a particular piece of equipment. I've connected 3 of 8 Digital Outputs available on PortA in parallel with the momentary pushbutton switches used for manual control. To stimulate one of the switches, I am pulling the digital line low for 100mS then returning it to a high (the other signals should remain high during this time). I scoped the signals and noticed very short (uS) +5VDC-Pk pulses occuring on the idle lines (also on the digital lines I'm not even using). I'm using a the "Write to Port" VI. These "blips" occur during the rising "and" falling edges of the active signal. Occasionally, these
"blips" are long enough to trigger the other two inputs that I'd like to remain inactive. Besides using a filter to reduce the inverse-peak of the unwanted signals, is there a way to programatically control or remove these "blips" alltogether? Or are these even supposed to be there?

[markwysong]

These blips can occur when writing to a digital port, and not masking the bits you do not wish to change.

If these blips are causing problems, take a look at the Write to Digital Port.VI. Double click on its icon to open its front panel. Go to its block diagram. You'll see where the port is configured on iteration 0, and then the Port Write is called, and there is an input to mask lines. This is what you'll need to do--put a value on this input to mask lines you want to remain the same. This should fix the blips you are seeing.

Also, if you are not manipulating more than one digital line at a time, you could use the Write to Digital Line.VI, which would accomplish the same thing. But if you need to update more than one line at the exact same ti
me, this method won't work for you.

Mark

[Spencer S.]

Doug,

I have one additional point concerning Write to Digital Port.vi. Make sure you correctly use the iteration terminal. If you do not wire anything to this terminal, the VI will configure the port during each iteration. This will also cause behavior similar to that which you are describing. If you are using this VI in a while loop, you can wire the loop's iteration terminal to the VI's iteration terminal. With this configuration, the port will be configured on the first iteration of the loop, but not on subsequent iterations.

Good luck with your application.

Spencer S.

[Doug1974]

Many thanks to Spenser S and MarkWysong for their suggestions.. It was the Iteration input that was causing my problems..

Thanks once again!