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 analog recording session of 10 seconds, with a sample rate of 10 samples a second, we would expect 100 samples.
- For a analog recording session of 60 seconds, with a sample rate of 10 samples a second, we would expect 600 samples.
However after writing some unit tests I was able to confirm:
- For a analog recording session of 10 seconds, with a sample rate of 10 samples a second, we got 76 samples.
- For a analog recording session of 60 seconds, with a sample rate of 10 samples a second, we got 481 samples.
I then tried reducing the sample rate:
- For a analog recording session of 10 seconds, with a sample rate of 1 samples a second, we got 11 samples.
- For a analog recording session of 10 seconds, with a sample rate of 1 samples a second, we would expect 10 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 analog input. The prototype for the function will look something like this:
__declspec(dllexport) signed long readAnalog(const char* deviceId, int line, int sampleRate, int numberOfSamples, DOUBLE_CALLBACK analogReadingCallback, 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.
- analogReadingCallback, 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 readAnalog function is:
- The function should be able to read at a given sample rate, for example 25 times per second.
- 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.
//
readAnalog(){
while shouldRead() {
doAnalogRead()
for each reading {
analogReadingCallback(reading);
}
if shouldRead() {
sleep()
}
}
}
doAnalogRead(){
DAQmxCreateTask("",&taskHandle);
DAQmxCreateAIVoltageChan(taskHandle, analogChannelId, analogChannelName, DAQmx_Val_Cfg_Default, -10.0, 10.0, DAQmx_Val_Volts, NULL);
DAQmxCfgSampClkTiming(taskHandle, "", 1000, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, 0);
DAQmxStartTask(taskHandle);
DAQmxReadAnalogF64(taskHandle, numberOfSamples, 10.0, DAQmx_Val_GroupByChannel, data, numberOfSamples, &numberOfSamplesRead, NULL);
DAQmxStopTask(taskHandle);
DAQmxClearTask(taskHandle);
}
In the future I will need to extend my c library to provide functions for writing DigtalOutput. As these functions will be in a library it is not possible to run the reading sessions a C program that will stopped.
I should probably also mention that in my first draft of the program I tried to create named tasks based on what type of reading was happening and on which channel/line. Prior to doing each reading I would then check to see if the task already exists, if it does, load it, stop it, and the clear it and the re-create it. This didn't seem to work. I kept getting errors about tasks with duplicate names. I then resorted to just give every task a name of "". I still have to go through the check it is exists, stop it, clear it, re-create it routine but it seems slightly more robust. Am I missing something here?
Thanks in advance for any help you can provide.
Cameron
