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 (https://www.ni.com/docs/en-US/bundle/usb-6008-6009-feature/page/pfi-zero.html) 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 https://www.ni.com/docs/en-US/bundle/labview-api-ref/page/vi-lib/waveform/dwdt-llb/analog-to-digital...
Regards,
William R.
National Instruments
William R.
