DAQmx Change Detection Error

[patrick.wright]

Hello,

I am having an issues with digital change detection using DAQmx with an NI-9401 digital I/O module.  I am setting up my data collection as follows

And then waiting for a rising edge using the following:

This used to be working perfectly fine until I physically moved my setup to a new location after which I now receive the following error every time I attempt to wait for an input change "Digital input detected a new sample clock before the previous sample was latched into onboard memory."  my digital input is being generated by a 555 timer in a monostable configuration which generates a ~10ms pulse.

I looked at the output of my 555 in a scope and do notice there are about 10ns of transients before the voltage stabilizes for the 10ms pulse.  This led me to think that I could simple slow down the sample clock of the DAQ device somehow so that it doesn't pick up these false logic changes.  However, I cannot figure out how to accomplish this.

Does this look like it might be the issue or could there be another problem?

Thanks!

[Kevin_Price · Accepted solution]

I think it's likely the transients are the problem.  The error text sounds like a problem at the hardware level, dealing with the circuitry for latching samples and rearming.   Apparently, during that transient time, the signal registers multiple digital transitions faster than the hardware can handle.

To further diagnose, try feeding in a cleaner pulse signal. 

-Kevin P

[patrick.wright]

It was definitely the transients causing the problem.  I added an additional 100uF filtering capacitor to the output of the 555 and that fixed the issue.  Perhaps the longer wire I attached the device to was picking up noise.

[AnalogKid2DigitalMan]

Don't digital input modules that support change detection also support filtering for minimum active high time?

(I don't have any hardware on-hand to check).

-AK2DM