PFIO Signal frequency

[obsessed]

Hello

I am currently using a USB 6008 to measure the displacement of a rotating shaft.  As a part of this I use PFIO and a tach to trigger the start of my reading such that all the readings begin at the same point on the shaft.

I also have use for displaying the tach frequency and the tach waveform, but I have had trouble accessing the data on PFIO.  Is it available for use in this manner?  Am I better off just to jumper the signal over to the analog side as well?

I can post code as neccesary but I really don't have any to post since I have not been able to use PFIO for anything but the trigger to date.

Thanks

TK

[GiantDeathRobot]

Hello TK,

This depends on what kind of signal the tach is outputting, but most likely this is beyond the abilities of the USB-6008. Based on the user guide section on PFI0 (Page 21 http://www.ni.com/pdf/manuals/371303n.pdf) it looks like PFI0 can either be used as a trigger, or as a counter input. So if the tach is putting out a pulse train, and counting edges in a period of time gives you the speed, then you would be out of luck because you would have to reconfigure the line, which would miss samples after the trigger has come in.

If it has another form of digital waveform, the USB-6008 is limited to software timed digital input. Timing on software timed inputs are not guaranteed, so you would be looking at a max sample rate of somewhere around 950-1000 Hz, and could occasionally drop samples. If the signal coming from the tach is lower than that, you should be able to read it using software timed digital I/O. Any faster than that, and you would need an X-Series USB DAQ device to do both the triggering and the hardware timed digital input.

Putting the signal on an analog input could give you access to the waveform, but you would then need to post-process your analog signal into a digital signal, which would add some overhead to your system. It would be less than ideal, but it might at least work. (Something up the lines of http://zone.ni.com/reference/en-XX/help/371361H-01/lvwave/analog_to_digital_wf/)

Regards,

William R.

National Instruments