Hi Frank,
I took a look at the code in DAQmx read position.vi, and it looks like an output of 80 or above from the 'samples available' output should be possible. The 'samples available' output just shows the total number of samples acquired (per channel) minus the current read position. Here is some pseudocode of what the DAQmx read position.vi is actually doing:
Let samples to read = 80;
IF (have not read in 80 new samples from current read position) {
leave current read position alone and keep reading in values;
ELSE (if you have read in 80 or more new samples from current read position)
set a new current read position that is 80 samples behind the most recent sample acquired;
The software is checking the hardware buffer for the number of samples available. When the VI gets called, if the DAQ card has not read in 80 new samples since the last time the current read position was set, it does nothing and waits for the next iteration of the loop. If it reads the buffer on the next iteration and finds that there are 100 samples waiting, it sets the current read position to 20 so that the DAQmx read will grab the most recent 80 samples, and disregard the other 20.
It looks like this VI was designed for the case where the DAQ card is acquiring data faster than the software can read it out of the buffer. If this were the case, the buffer would eventually fill up and then throw an error. This DAQmx read position VI makes sure that when the buffer starts filling up past the number you are trying to read, you always grab the most recent samples. For your specific case, I'm guessing that your program is reading the samples out of the buffer fast enough, so the 'samples available' output should never hit 80 or above.
That was a little tough to put down into words...let me know if any of this is unclear.
Thanks,
Justin M.
National Instruments
