Channels—Physical Versus Virtual

A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal, as in the case of a differential analog input channel or a digital port of eight lines. Every physical channel on a device has a unique name (for instance, SC1Mod4/ai0, Dev2/ao5, and Dev6/ctr3) that follows the NI-DAQmx physical channel naming convention.

Virtual channels are software entities that encapsulate the physical channel along with other channel specific information—range, terminal configuration, and custom scaling—that formats the data. To create virtual channels, use the DAQmx Create Channel function/VI or the DAQ Assistant.

Virtual channels created with the DAQmx Create Channel function/VI are called local virtual channels and can only be used within the task. With this function/VI, you choose the name to assign for the virtual channel, which is used in the rest of the NI-DAQmx software framework to refer to the physical channel.

If you create virtual channels with the DAQ Assistant, you can use them in other tasks and reference them outside the context of a task. Because these channels can apply to multiple tasks, they are called global virtual channels. You can select global virtual channels with the NI-DAQmx API or DAQ Assistant and add them to a task. If you add a global virtual channel to several tasks and modify that global virtual channel with the DAQ Assistant, the change applies to all tasks that use that global virtual channel.

The following example illustrates the difference between physical and virtual channels.

Problem: Create an NI-DAQmx virtual channel to measure temperature in the range 50° C to 200° C using a J-type thermocouple wired to channel 0 on an M Series device configured as Device 1. Use LabVIEW or LabWindows/CVI to write your application.

Solution:

  1. Use the Create AI Thermocouple Channel function/VI.
  2. Use Dev1/ai0 as the physical channel on the device to which the thermocouple signal is connected.
  3. Specify myThermocoupleChannel as the name to assign to your virtual channel.
  4. Select the appropriate values for the thermocouple type and range inputs. NI-DAQmx applies these attributes to the virtual channel.

You have now created a virtual channel.



See Also

Device-Specific Physical Channels

Device-Specific Internal Channels