SPI bus timing, 1ms repeat acquisition

Question

I plan to use digital I/O to create a SPI bus to read the output from a sensor.   I need to repeat a 2 byte read at a 1ms interval.  I was hoping to use a USB-8451 for this but I am unaware how I can control the 1ms repeat timing using LabVIEW.  If I only need 100ms of data perhaps I can use the NI845x SPI Write Read.vi to read an array of 200 bytes and then divide the array up into 100 2-byte pieces.  Depending on my sensor, I may need to deselect the Chip Select between the 100 reads to permit the DUT to update its data.
Alternatively, I have considered the USB-6221,6225,6229,6251,6259,6281 or 6289.  These seem to have better hardware timing features but may be limited to 2047 samples.  I assume that this means 256 bytes which could give me 128 samples from the sensor.
Thanks for any suggestions you can provide.

Answer 1

Hey Ryan, 
A picture of the behavior you are seeing would be helpful. The NI 9411 should only be reading 50 bits every 1 ms.  
It may not be possible to read 25 bits (do you mean 50 bits? 25 high 25 low) and push it to a queue without encountering an overflow error. If you take a look at the above code the digital input will receive 50 sample clocks every 1 ms. This is equivalent to acquiring 50 points every 1 ms which is an acquisition speed of 50 samples/1 ms=50 kHz. The read loop must keep up with the 50 kHz rate otherwise the buffer will overflow. In the above example I’ve set the read to pull 5000 samples (x) with the assumption that the loop will take less than or equal to .1 seconds (y). This yields a software acquisition speed of 50 kHz (5000 samples/100 ms). If the loop speed is faster than 100ms then the 10 seconds timeout on the DAQmx read will allow for the read to block so the FIFO may be filled.
The options available for question 2 are below. They may be used separately or in conjunction.
Move the DAQmx Read for the NI 9411 to its own independent while loop, set the DAQmx Read to acquire 50 samples, do not graph the data, and pass the data to a Queue for processing in a consumer loop. This will increase the loop speed which may allow you to keep up with the 50 kHz acquisition speed. This may not work because the loop speed will need to be 1 ms or less.
Increase the value of the Samples per Channel control that goes into the DAQmx Timing VI. This will increase the DAQmx Software Buffer size. This buys time until you receive an overflow error because the DAQmx Software Buffer is being filled faster than samples are removed.
Read in 5000 sample chunks (producer loop), push this to the queue, and perform the analysis in 50 bit chunks (consumer loop). The additional queue created should allow for the acquisition loop to keep up.
Regards,
Izzy O.
Product Support Engineer
ni.com/support