@Dennis_Knutson wrote:
As you can see from the specs, the digital lines are software timed. The ms rate you are getting is about the best you can hope for and with windows, that will be subject to considerable jitter.
The digital lines are software timed, true. But why do you think that 1 ms is the fastest that software timed IO can operate? Do you have any evidence or reference for this? In my opinion that statement is naive.
Open up your favorite programming environment and write a simple loop. Using .Net on a modern PC (e.g. 1.6 GHz+) you'll see that the loop runs MILLIONS of times per second. I benchmarked my fairly slow PC at 7 million loops per second. That is 7000 loops PER MILLISECOND! Now add in some function call that communciates with the hardware, for example AnalogMultiChannelReader.ReadSingleSample(). Even with this function call (accurately returning the correct voltage) the loop operates at 4MS/s -- i.e. 4000 samples per millisecond.
So is digital output limited by software execution speed? NO!
Is digital output limited by USB communication speed? NO!
The 1 ms limit for digital output is NOT some jittery approximate speed. It is a very precise, controlled timing. To see this, place a function call that performs digital output in your benchmark code, e.g. using the following function: DigitalSingleChannelWriter.WriteSingleSamplePort
If you benchmark a loop in which you use that function but write the SAME value to the port (e.g. 0) over and over, you will see that runs at about 100 kS/s (i.e. about 100 times per millisecond). I'm guessing this is because the NiDaqmx driver realizes that the output is already set to the specified value and does not lock the thread for the 1 ms timeslot.
However if you write alternate values to the port, such as:
while(time < 1000)
{
writer.WriteSingleSamplePort(false, 1);
writer.WriteSingleSamplePort(false, 0);
}
You will see that this operation executes at ~500 loops per second, which is exactly 1000 writes per second. Moreover, if you time each function call individually, you will see that they consume EXACTLY ONE millisecond each.
(Note that in your own testing, you may see anywhere from 490 to 500 loops per second, depending on processor load, but this is only because the thread manager could devote time slices to another thread during the benchmark. But if you are careful to measure only the time consumed by the digital output function calls, you will see that they are exactly 1 millsecond each).
So in summary, both software execution and USB communication occur at well over 1MS/s. Function calls to write the SAME digital output value to a port execute at about 100 kS/s, but writing alternate on/off values (to a single line or to a whole port, it doesn't matter) occurs at precisely 1 kS/s (i.e. 1 ms each).
So this 1 ms limit IS NOT an operating system or software execution timing issue. My guess is that it is imposed by the NIDAQmx driver, and may be specific to the device being used.
With a device that is hardware timed, you can certainly avoid this issue. But that in itself doesn't mean that software timed IO needs to be limited to 1 ms. (Indeed analog input can easily be performed at up to 48 kS/s).
Does anyone have a reference that explains the precise 1 ms delay when writing digital output? Does anyone have a reference regarding the maximum sampling frequency of digital IO on the USB-6008/9 ? (The sampling frequency for analog input is given in the specifications, but NOT the digital sampling frequency)
Thanks!
.) The ints are output at the rate of approximately 1 per millisecond and I need it to go much faster than this - maybe 100x or 1000x faster. I've googled and found mention of Task.Timing.ConfigureSampleClock(), but as far as I can see this applies to Analog I/O and I don't see a way to make it affect Digital Outputs.
