Filtering with PCIe-6536 change detection

[dhoepner]

I have a PCIe-6536 that I have setup for change detection (using the NI sample change detetction VI).  The inputs are connected to an absolute encoder.  I am getting some bad counts and am concerned that there may be some noise on the inputs that are being captured by the 6536 change detection.  I am looking for a way to add some filtering so that any short noise pulses are ignored.  I have not been able to figure out how to add this filtering.  Is it possible?  If so, how would I go about doing this.

[Josh_B1]

Hi dhoepner,

The PCIe-6536 does not have the ability to filter signals. I have a few suggestions for you. First I would check how you have your change detection configured. Below is the manual for the PCIe-6536. 

http://www.ni.com/pdf/manuals/374373e.pdf

If you refer to page 11 you will see the various configurations for change detections. If the count you are seeing is twice what you would expect then perhaps to have it configured for both rising and falling. If you suspect a noisy signal you can hook up an oscilloscope to the output of the encoder. This would allow you to verify the signal that the encoder is outputting. How do the counts you receive differ from the count you expect? Also, what is the frequency the encoder is outputting at?

Regards,

Josh Brown

[dhoepner]

Hello Josh,

We are looking at the outputs from an absolute encoder that counts up using gray code values.  What we see is just a few places per revolution of the encoder we get some counts that are not sequential.  It is behaving just like it would if there was noise.  We are looking at the actual values that get captured and everything is good except about 3 or 4 times per revolution of the encoder (for our test sample that is 360 values). 

We have checked it with a scope and everything looks perfect.  We have also attached a logic analyzer and captured a complete encoder revolution to a text file.  Analysis of this text file shows no problems.  We only see this with the change detection on the PCIe-6536.  Our assumption is that the NI card is faster and so capturing things that the analyzer is missing.

Also, we have a different type of encoder and test HW setup that works with the same vi that I am using to test with here.

[Josh_B1]

Hi dhoepner, 

What is the hardware set up that the change detection vi works with?

Regards,

Josh Brown

[dhoepner]

It looks like the attached diagram.  We have our own serial to parallel converter that we use.  Parallel encoders work, serial ones do not.  However, everything looks good with the analyzer and scope.

The hardware engineer has a few questions:

We do not see any noise on the incoming encoder signals.
1) Is it possible there is noise getting into some part of the NI 6536 card?
2) What notifies the system of a change?
3) Could there be noise on that notification bit?