I've played around with your code and I've been able to reproduce what you are seeing. It seems that the slow down is coming from trying to tristate a single line in a channel that has multiple lines. Fortunately, I have a few recommendations that fixes the issue. Which recommendation you pick will depend on your application. Are you going to want to tristate all of the lines at once, or individual lines?
All Lines at Once
Change
iStatus = DAQmxSetDOTristate(g_DOtaskHandle[0],"Dev0/port0/line0",1);
To:
iStatus = DAQmxSetDOTristate(g_DOtaskHandle[0],"",1);
This will tristate all the lines in the port at once.
One Line at a Time
This is more complex, because we need to change how we are defining channels in the task.
First, the following change is needed:
iStatus = DAQmxCreateDOChan(g_DOtaskHandle[0],"Dev0/port0/line0:7","",DAQmx_Val_ChanPerLine);
This will create a separate channel for each line.
When you call DAQmxSetDOTristate, you'll call it just like before.
The main change is that your DAQmxReadDigitalScalarU32 will need to become a DAQmxReadDigitalU32. The Write function will change from DAQmxWriteDigitalScalarU32 to DAQmxWriteDigitalU32.
What these changes have basically done is go from having a task with one channel, made up of 8 digital lines to having a task with 8 channels, each with 1 digital line.
My testing here has shown an order of magnitude decrease in the tristate time if either of these changes is made. The one you pick will depend on how your application needs to work. I'm also going to file a corrective action request to have a developer investigate when there would be an order of magnitude increase in the time it takes to tristate a single line if you use the DAQmx_Val_ChanForAllLines argument.
Hope this helps!
Seth B.
Principal Test Engineer | National Instruments
Certified LabVIEW Architect
Certified TestStand Architect
