make USB-6001 digital output always high or low in C

Hello all,

I am new to the NI DAQ interface. I have an USB-6001 and I am trying to use this device to control some logic circuit in C. So what I want to do is:

* set some digital output lines to high or low levels, and change their status when needed (in C).

I have tested the device in NI MAX -> Test Panels, and found that the device is able to do this. Then I try to do it in C. I hace checked the examples, and the function I should use is the one called "DAQmxWriteDigitalU32". I have problem in understanding its input parameters. I have tried something with my own understanding, but it does not work as I expected. Here is one test I did:

uInt32   data=1;

int32 written;

TaskHandle taskHandle=0;

DAQmxErrChk (DAQmxCreateTask("",&taskHandle));
DAQmxErrChk (DAQmxCreateDOChan(taskHandle,"Dev1/port0/line7","",DAQmx_Val_ChanForAllLines));
DAQmxErrChk (DAQmxStartTask(taskHandle));
DAQmxErrChk (DAQmxWriteDigitalU32(taskHandle,1,1,10.0,DAQmx_Val_GroupByChannel,&data,&written,NULL));

taskHandle=0;
DAQmxErrChk (DAQmxCreateTask("",&taskHandle));
DAQmxErrChk (DAQmxCreateDOChan(taskHandle,"Dev1/port0/line0","",DAQmx_Val_ChanForAllLines));
DAQmxErrChk (DAQmxStartTask(taskHandle));
DAQmxErrChk (DAQmxWriteDigitalU32(taskHandle,1,1,10.0,DAQmx_Val_GroupByChannel,&data,&written,NULL));

I want to simply set "Dev1/port0/line7" and "Dev1/port0/line0" to high level, but only "Dev1/port0/line0" responds me. The second parameter of the function DAQmxWriteDigitalU32 corresponds to numSampsPerChan. If i replace it (currently 1) with a larger value, e.g. 100, I can see that "Dev1/port0/line7" sends a number of 1 out, then return back to 0. So I guess the problem is just that I do not understand well all parameters of the function DAQmxWriteDigitalU32. Can anyone please tell me how I can set a digital output line to 1 or 0? 

Thanks!

Hongkun