pci-6624 task synchronisation

[josswern]

Kevin,
you're surely right with what you say about A&B channel states etc., but in my test setup, I just feed ALL A&B, z-index etc. signals from one physical encoder to all of my counters,
so I assume they should all count the same position values (which they do, in fact, but not at the same data index, there ist always some samples phase difference from counter to counter).
then you mention other counters which can perform triggered position measurment - do you have one in mind (which can handle quadrature encoder signals, of course).
maybe I can get my 6624 replaced by NI, as it just doesn't do what it was expected to.
thanks.

[Kevin_Price]

The 6602 can perform sync'ed position measurements using an "Arm Start" trigger which in LabVIEW can only be found using the more low-level DAQmx Trigger property node.  This is different from the standard DAQmx Start Trigger call that is used for many other measurements.  The M-series multifunction boards' counters can also do this.

My understanding is that the 6624 is a close cousin of the 6602 but with some additional circuitry to support larger voltages (and perhaps some other things...).  I would expect it to be similarly capable of sync'ing 3 position measurements with an "Arm Start" trigger.  The only online docs I could find for the 6624 was the general TIO Help, which vaguely referenced the existence of the "Arm Start" trigger, so I would be cautiously optimistic.

Point taken re: splitting 1 encoder's signals out to 3 different counters.  That hadn't registered with me when I first read the thread.

-Kevin P.

[josswern]

Kevin,
I also use the 6602 for certain applications, but to my understanding, this one cannot handle A/B+/- signals from position encoders directly, as it has only source/gate input for each counter, up/down as a separate line extra, but this one has to be generated by extra circuitry from encoder signals, which has proven to be unreliable (at least for me 🙂 )
as opposed, the 6624 nicely handeles the encoder signals nicely without extra hardware, but, as said, lacks external triggering and exact counter syncronisation.
we have now decided to feed a common reference signal (z-index) to all counters and correct the measured data later for the mesured phase shift.
I hope this will work sufficiently well.
again, thanks to all who contributed to this thread.

[Kevin_Price]

Not looking to pester, but I'm not sure you've understood what I've suggested about the "Arm Start" trigger that is a unique type of triggering used to sync counter input tasks.  It is distinct from "normal" digital edge triggering.

Here is a link to an example that uses an Arm Start trigger for a counter input task, and the info page says that it's compatible with the 6624 hardware.  That makes me pretty confident that you CAN sync the 3 encoder tasks on your 6624.  I don't know the CVI syntax for it, but the board and DAQmx driver seem to support it.

(For future reference, the 6602 *can* handle true quadrature.  I've used it that way extensively.  When in position measurement mode, the Source input is channel A, the UP/DN (aka AUX) input is channel B, and the Gate input is the Z-index pulse.  The counting mode I think you were describing (Source edge counting with direction determined by digital state of UP/DN input) is a different measurement mode which is indeed inferior.)

-Kevin P.

[josswern]

ok,
I just admit I didn't look at the "arm start" feature so far, and as I don't use LV, I also can't look at the example 😞
I'll go through my CVI daqmx library selection once again when I get the 6624 back (we already started actual measurments using my beforementioned workaround).

as for the 6602, I just wasn't aware this can also measure encoder position directly (as it doesn't have input lines for each signal's '-' track, whereas the 6624 has),
maybe I also missed/misunderstood the manual.