Help with Basic Configurat​ion of Parameters for NIDAQmx

Question

Hey all,
I'm struggling to get data from my DAQ at the rate I expect. I've read through the scattered documentation I've been able to find, but it does not behave as I expect.
I simply want to have a loop that continually reads the latest sample from the DAQ and publishes it (using the robot operating system). Unfortunatelly, I'm seeing delays on the order of many seconds between when events occur and when they show up from the DAQ. I suspect that events are stacking up in the buffer faster than I'm reading them. How should I deal with this?
// Timing parameters
char clockSource[] = "OnboardClock";
// Data read parameters
#define bufferSize (uInt32)1
float64 data[bufferSize * channelCount];
// how many points to read per channel
int32 pointsToRead = bufferSize;
int32 pointsRead;
// timeout in seconds... should be even smaller than 1.0s
float64 timeout = 10.0;
int32 totalRead = 0;
bool isHigh = false;
adc_msgs::ADCSignal adc;
DAQmxErrChk(DAQmxBaseCreateTask("", &taskHandle));
DAQmxErrChk(
DAQmxBaseCreateAIVoltageChan(taskHandle, channels.c_str(),"",DAQmx_Val_Cfg_Default,min,max,​DAQmx_Val_Volts,NULL));
// The sampleRate used here determines how fast samples are read on the hardware side into its internal buffer
DAQmxErrChk(
DAQmxBaseCfgSampClkTiming(taskHandle, clockSource, sampleRate, DAQmx_Val_Rising, DAQmx_Val_ContSamps, 1));
DAQmxErrChk(DAQmxBaseCfgInputBuffer(taskHandle, 200000));
DAQmxErrChk(DAQmxBaseStartTask(taskHandle));
while (ros::ok()) {
DAQmxErrChk(
DAQmxBaseReadAnalogF64(taskHandle,pointsToRead,tim​eout,DAQmx_Val_GroupByScanNumber,data,bufferSize*c​hannelCount,&pointsRead,NULL));
adc.header.stamp = ros::Time::now();
adc.signal.clear();
if (meta != NULL){
//ROS_INFO('Meta: %s', meta->data.c_str());
adc.meta = meta->data.c_str();
for (int i = 0; i < channelCount; ++i) {
float32 val = (float32) data[i];
adc.signal.push_back(val);
// here for debugging
if (i==0) {
if (!isHigh && val > 1.0) {
isHigh = true;
printf("going high\n");
if (isHigh && val < 1.0) {
isHigh = false;
printf("going low\n");
pub.publish(adc);
ros::spinOnce();
// this ensures we sleep for the appropriate amount of time so that the loop rate is
// sampleRate
loop_rate.sleep();

Answer 1

I think maybe I have a misunderstanding of how this should work.
Say I want to sample at 1kHz, so I need to get a value every 1ms. Can my while loop not just grab the latest value from the channel? Ideally, I think it would make sense if I used a higher sampleRate than the actual rate at which I want data, say 2kHz, and a buffer that only stored one sample, so that the buffer always contained the latest value. Everything I've tried to get this kind of behavior hasn't worked.
Regarding your suggestion:
If I use this buffered system, won't I end up getting burst behavior? So, as you propose I would try to read in 250 samples on each iteration. So that means in one second I would end up burst publishing 250 samples 4 times.
Is that the kind of behavior I should expect from this sort of system? Is there no way to do a non-blocking call when reading?
I've tried to do an on demand loop, but when I do that the best I can get is about 300Hz on my system. This is very surprising to me as audio recording devices, which work over USB, sample above 44kHz, with low latency. Are they in fact working in a bursting fasion as well or are the devices just designed better?