Quadrature counter dividing and triggering

Question

I'm currently looking into using an NI USB 6353 device in a project, and I'm confused about its capabilities. Here is what I'm trying to achieve:
1. Reading an A/B (no Z) quadrature input to increment/decrement a counter (preferrably in X4 mode).
2. Produce a digital output signal with transitions on each counter increment/decrement.
3. Frequency dividing the counter transitions to produce a digital signal used to trigger various external hardware. The divider should be specifiable (probably around a few thousand).
4. Using another frequency divided version of the counter to trigger simultaneous analogue/digital sampling on the device.
5. Have the ability to make the counter time based (just incrementing at a set frequency), rather than using the quadrature input. The resulting signals should all still be divided and output correctly.
Being able to use the digital outputs on the device independently (on demand, timing isn't particularly important here) would be great, too.
Is this possible using the DAQmx C or C# API? Are there any other devices I should be looking at?

Answer 1

Hi Matthew,
Matthew S wrote:
Wouldn't the change detection mean that I couldn't sample the digital input lines at a divided rate? Or can I simply implement the change detection using a single DIO port and still have the remaining DI lines to use?
I'm glad you brought it up--I guess I overlooked that you were planning to use clocked DI.  If you're using DI change detection you cannot sample the DI lines off of another clock source.  Unfortunately, port 0 is the only port that supports change detection (even though the manual currently implies otherwise) as well as the only port that supports clocked DIO.  You can still do static DI on other lines, but the clocked DI subsystem can only have one timing source.  You can always oversample your DI using the change detection, then index out 1 out of every N samples in software (the DI should be good at rates of up to at least 1 MHz continuously).
Matthew S wrote:
It sounds like the X-Series can do what we need. We'll probably stick with the 6353, because we need at least 20 analogue inputs, and sampling rates of at least 1MSps.
Keep in mind the 6353 is multiplexed--the aggregate rate is 1 MHz, but sampling 20 lines at the same time would only give you 50 kHz per channel.
Matthew S wrote:
I'm slightly confused as to how to use the change detection to implement what we want. Essentially, we'd like to take the X4 quadrature input and convert it to a step signal (as in, step/direction) that will be used to control a hardware variable antialiasing filter. Will this require another counter? If this can't easily be done on the USB DAQ device, we can do it using external hardware.
You can get the step from your x4 encoder with change detection easily enough, but I don't think there is a good way to get the direction as an external hardware signal.  I think the better option is to use a flip flop to latch the encoder state (either latch A using B as the clock or vice-versa) to obtain the direction.
Matthew S wrote:
How hard is it to get the direction and also speed (or frequency) of the
original quadrature input for each AI and DI sample taken? I assume
that the frequency measurement would require another counter? How about
direction?
If you're using a latching circuit to output the direction, you could just sample this off of one of the DI lines (assuming you have one available).  To measure the frequency of the encoder, I suggest looking into the method described in Figure 7-15 of the X Series User Manual to use another counter to measure the frequency of either A or B.  During each sample clock period the counter counts the number of encoder pulses as well as the number of internal timebase ticks that occurred during those pulses.  The driver uses this information to return the average frequency of your signal in between sample clock pulses.
So... you were right to have some concerns about using change detection alongside clocked DI.  You'll have to pick one or the other--I think I'd prefer to oversample DI and throw away samples, depending on the maximum frequency coming from your encoder.  Also of concern is the number of counters (there are 4 available on the board).  In addition to the 4 counters, keep in mind taht there is a Frequency Generator that can be used to output specific frequencies:
    {20 MHz, 10 MHz, 100 kHz} / {1:16}
Please don't hesitate to post back if you have any questions--there's alot you need to do so you might have to make some compromises.  We're starting to get into territory where an FPGA might be more suitable for your digital requirements.
Best Regards,
John Passiak