DAQmx ANSI C read Digital Input using NI-USB-6008

[cdouglas]

Hi Folks,

I have a NI DAQmx ANSI C problem which I hope someone will be able to help me with. Before I delve in to the problem I should state that I have a NI-USB-6008 device, which will be running on Windows 7, with which I wish to read digital input and analog input voltage. I will be writing an ANSI C library that will be compiled as a DLL using Microsoft Visual Studio 2010. The library will then be invoked via Java using JNA. I am an experienced Java programmer who is fairly comfortable programming in C. I have looked at the examples provided in C:\Programme\National Instruments\NI-DAQ\Examples\DAQmx ANSI C\ and searched the forums, but havent found what I'm looking for yet. Appologies if this post is not in the correct forum. 

I have a program which does most of what I need it to, but I am getting some unexpected results.

When calling the functions I'll outline below here are the results I am getting:

• For a digital recording session of 10 seconds, with a sample rate of 10 samples a second, I would expect 100 samples. 
• For a digital recording session of 60 seconds, with a sample rate of 10 samples a second, I would expect 600 samples.

However after writing some unit tests I was able to confirm:

• For a digital recording session of 10 seconds, with a sample rate of 10 samples a second, I got 95 samples.
• For a digital recording session of 60 seconds, with a sample rate of 10 samples a second, I got 593 samples.

I then tried reducing the sample rate:

• For a digital recording session of 10 seconds, with a sample rate of 1 samples a second, I would expect 10 samples.
• For a digital recording session of 10 seconds, with a sample rate of 1 samples a second, I got 11 samples.

From these results I suspect that the problem lies in having to use sleep to provide the sampling rate that I am interested in.

I should point out that I have no issues with the Java integration, that seems to work fine.

Is there any NI DAQmx functions for the NI-USB-6008 device which will prevent me from calling sleep every n milliseconds?
Am I using the correct API calls?
Is my NI-USB-6008 device capable of doing what I need it to?

I want to expose a function which will read digital input. The prototype for the function will look something like this:

__declspec(dllexport) signed long readDigital(const char* deviceId, int line, int sampleRate, int numberOfSamples, INT_CALLBACK digitalReadingCallback, SHOULD_READ_CALLBACK shouldReadCallback, LOG_CALLBACK logCallback);

where:-

• deviceId, the device id is the id of the NI-USB-6008, eg Dev0.
• line, the numeric identifier for the particular pin on the device.
• sampleRate, how many times per second to perform a reading.
• numberOfSamples the number of samples to take upon each reading.
• digitalReadingCallback, a callback function which should be called with each value that is read.
• shouldReadCallback, a callback function which should be used to establish if measurements should still be made.
• logCallback, a callback which can be used to pass log messages back to the Java program.

The desired behaviour for my readDigital function is:

• The function should be able to read at a given sample rate, for example 25 times per second.
• The function should should only invoke the digitalReadingCallback upon a change in the value from the digital input port.
• It should be possible to stop the function reading from Java via means of the shouldReadCallback.

My current implementation involves:

//

// Please not that this is the jist of the code that I am running some code has been missed out to try to improve the readability for this post.

//

readDigital(){

DAQmxCreateTask("",&taskHandle);

DAQmxCreateDIChan(taskHandle,digitalLineId,digitalLineName,DAQmx_Val_ChanForAllLines);

DAQmxStartTask(taskHandle);

while shouldRead() {

DAQmxReadDigitalLines(taskHandle,numberOfSamples,10.0,DAQmx_Val_GroupByChannel,data,dataSizeInBytes,&numberOfSamplesReadPerLine,&numberOfBytesPerSample,NULL);()

for each reading {

if we have no previous data {

digitalReadingCallback(value)

}

else{

if value is different to previous value {

digitalReadingCallback(value)

}

else {

do nothing

}

}

}

if shouldRead() {

sleep()

}

}

DAQmxStopTask(taskHandle);

DAQmxClearTask(taskHandle);

}

Thanks in advance for any help you can provide.

Cameron

[Dennis_Knutson]

As you can see in the specs for the 6008, the digital I/O is software timed. You simply cannot specify a sample rate. You take one sample at a time and expect some serious jitter in the sample rate due to windows.