Data Acquisition Toolbox

Adding Channels to an Analog Input Object

After creating the analog input object, you must add hardware channels to it. As shown by the figure in Adding Channels or Lines, you can think of a device object as a container for channels. The collection of channels contained by the device object is referred to as a channel group. As described in Mapping Hardware Channel IDs to MATLAB Indices, a channel group consists of a mapping between hardware channel IDs and MATLAB indices (see below).

When adding channels to an analog input object, you must follow these rules:

• The channels must reside on the same hardware device. You cannot add channels from different devices, or from different subsystems on the same device.
• The channels must be sampled at the same rate.

You add channels to an analog input object with the addchannel function. addchannel requires the device object and at least one hardware channel ID as input arguments. You can optionally specify MATLAB indices, descriptive channel names, and an output argument. For example, to add two hardware channels to the device object ai created in the preceding section:

• chans = addchannel(ai,0:1);
  

The output argument chans is a channel object that reflects the channel array contained by ai. You can display the class of chans with the whos command.

• whos chans
    Name        Size         Bytes  Class
  
    chans       2x1            512  aichannel object
  
  Grand total is 7 elements using 512 bytes
  

You can use chans to easily access channels. For example, you can easily configure or return property values for one or more channels. As described in Referencing Individual Hardware Channels, you can also access channels with the Channel property.

Once you add channels to an analog input object, the properties listed below are automatically assigned values. These properties provide descriptive information about the channels based on their class type and ID.

Table 4-2: Descriptive Analog Input Channel Properties 

Property Name	Description
HwChannel	Specify the hardware channel ID.
Index	Indicate the MATLAB index of a hardware channel.
Parent	Indicate the parent (device object) of a channel.
Type	Indicate a channel.

You can display the values of these properties for chans with the get function.

• get(chans,{'HwChannel','Index','Parent','Type'})
  ans = 
      [0]    [1]    [1x1 analoginput]    'Channel'
      [1]    [2]    [1x1 analoginput]    'Channel'
  

If you are using scanning hardware, then the MATLAB indices define the scan order; index 1 is sampled first, index 2 is sampled second, and so on.

Note    The number of channels you can add to a device object depends on the specific board you are using. Some boards support adding channels in any order and adding the same channel multiple times, while other boards do not. Additionally, each channel might have its own input range, which is verified with each acquired sample. The collection of channels you add to a device object is sometimes referred to as a channel gain list or a channel gain queue. For scanning hardware, these channels define the scan order.

Creating an Analog Input Object

Referencing Individual Hardware Channels