Need help with digital output on NI USB-6501 (why does writing bits on one port affect bits on another port?)

Hi,

I'm starting writing code (in C) for the NI USB-6501 and it seems like I'm facing a situation where writing to lines on one port will affect lines on another port.  I'm probably doing something wrong or there is some fundamental channel/port/task concept I don't understand...  I tried searching the forums for an explaination but with no success.

Details:

What I am trying to acheive is simple: I want to either set the 3 first bits of port0 or the 3 first bits of port1.  When I set the bits on one of the ports, I want the bits on the other port to stay the same.  So for example, at one point my application will get executed to set the port0 lines to 00000000, then a bit later my application is executed again but this time to set the port1 lines to 00000001 (but I want to keep port0 as-is).

So when I write the 3 first bits of port 0, all good, the bits on port 0 are set correctly (they are all low).

But then when I write the 3 first bits of port 1, the 3 first bits of port 1 are set correctly ** but the 3 first bits of port 0 are also affected and they all change to 1's?! **

Is this normal?

My code looks like this (it does not return any errors so I removed the error handling code for clarity):

void write_to_port_0(uInt8 bits)
{
// Task parameters
int32 error = 0;
TaskHandle taskHandle = 0;
char errBuff[2048];

// Channel parameters
char chan[64];

// Write parameters
uInt8 w_data [1];
int32 written;

sprintf(chan, "Dev1/port%i/line0:3", 0);
printf("Channel: %s\n", chan);

// Create Digital Output (DO) Task and Channel
error = DAQmxBaseCreateTask ("", &taskHandle);
printf("TaskHandle: %i\n", taskHandle);

error = DAQmxBaseCreateDOChan(taskHandle,chan,"",DAQmx_Val_ChanForAllLines);
error = DAQmxBaseStartTask (taskHandle);
w_data[0] = bits;

error = DAQmxBaseWriteDigitalU8(taskHandle,1,1,10.0,DAQmx_Val_GroupByChannel,w_data,&written,NULL);
if (taskHandle != 0)
{
DAQmxBaseStopTask (taskHandle);
DAQmxBaseClearTask (taskHandle);
}

return;
}

void write_to_port_1(uInt8 bits)
{
// Task parameters
int32 error = 0;
TaskHandle taskHandle = 0;
char errBuff[2048];

// Channel parameters
char chan[64];

// Write parameters
uInt8 w_data [1];
int32 written;

sprintf(chan, "Dev1/port%i/line0:3", 1);
printf("Channel: %s\n", chan);

// Create Digital Output (DO) Task and Channel
error = DAQmxBaseCreateTask ("", &taskHandle);
printf("TaskHandle: %i\n", taskHandle);

error = DAQmxBaseCreateDOChan(taskHandle,chan,"",DAQmx_Val_ChanForAllLines);
error = DAQmxBaseStartTask (taskHandle);
w_data[0] = bits;

error = DAQmxBaseWriteDigitalU8(taskHandle,1,1,10.0,DAQmx_Val_GroupByChannel,w_data,&written,NULL);
if (taskHandle != 0)
{
DAQmxBaseStopTask (taskHandle);
DAQmxBaseClearTask (taskHandle);
}

return;
}

Anyone else had this issue before?

Thanks,

  Andre