Hi Ed,
You should be able to make your reads execute much faster by configuring Implicit timing on the counter input task. Try using this:
DAQmxCfgImplicitTiming (taskHandle, DAQmx_Val_ContSamps , 1000); //Configures software buffer
DAQmxSetReadRelativeTo(taskHandle, DAQmx_Val_MostRecentSamp ); //Only read the most recent sample from the buffer
DAQmxSetReadOffset(taskHandle, 0);
DAQmxStartTask(taskHandle);
do
{
Delay( 0.010 );
t1 = Timer();
error = DAQmxReadCounterF64(taskHandle,1,0.001,&data[0],100,&count32,NULL); // Read 1 sample instead of -1 to avoid returning empty buffer
t2 = Timer() - t1;
printf("Delay: %.3f Error: %i Frequency: %.3f Hz\n", t2, error, data[0]);
} while ( ! KeyHit() );
Configuring timing allocates kernel memory for the hardware to transfer samples to as the data becomes available. Calls to DAQmx Read will read back samples from this kernel buffer rather than asking the hardware to send back the next measurement that it takes.
In your case I think you want to read back the most recent sample and ignore any samples you might have missed (in case your input frequency is higher than the loop rate), which is why I have set the ReadRelative property to "most recent sample". The default behavior (read relative to "current read position") is to attempt to read every sample that comes in and if the loop does not keep up you will receive an overwrite error.
The downside to this method is that you still cannot make the hardware tolerant to overwrites--every sample must be transferred from the hardware to the PC buffer successfully or you will receive errors (requires restarting the task to clear them). The hardware you are using does not have a dedicated on-board FIFO for the counters, so the device must transfer each sample as it comes in. Benchmarks have been made for these transfers to be sustainable to ~200 kHz-300 kHz or so (using DMA), so you'll probably be fine for the rates that you seem to be interested in.
One caveat is that the 6024E only has a single DMA channel and the 6602 only has 3, I'm not sure offhand if the buffered counter task will default to DMA or not. If you get any errors regarding the DMA channels being in use or otherwise not available, you can change the transfer mechanism to interrupts using DAQmxSetAIDataXferMech, but the maximum frequency you will be able to measure using interrupts would likely be only in the 10s of kHz (somewhat CPU dependant). This is still way faster than the frequencies you have mentioned so far but I wanted to point it out anyway just in case.
Best Regards,
John Passiak
