Data Acquisition Toolbox

Writing and Reading Digital I/O Line Values

After you add lines to a digital I/O (DIO) object, you can:

• Write values to lines
• Read values from lines

Note    Unlike analog input and analog output objects, you do not control the behavior of DIO objects by configuring properties. This is because buffered DIO is not supported, and data is not stored in the engine. Instead, you either write values directly to, or read values directly from the hardware lines.

Writing Digital Values

You write values to digital lines with the putvalue function. putvalue requires the DIO object and the values to be written as input arguments. You can specify the values to be written as a decimal value or as a binary vector (binvec). A binary vector is a logical array that is constructed with the least significant bit (LSB) in the first column and the most significant bit (MSB) in the last column. For example, the decimal value 23 is written in binvec notation as [1 1 1 0 1] = 2⁰ + 2¹ + 2² + 2⁴. You might find that binvecs are easier to work with than decimal values because there is a clear association between a given line and the value (1 or 0) that is written to it. You can convert decimal values to binvec values with the dec2binvec function.

For example, suppose you create the digital I/O object dio and add eight output lines to it from port 0.

• dio = digitalio('nidaq',1);
  addline(dio,0:7,'out');
  

To write a value of 23 to the eight lines contained by dio, you can write to the device object.

• data = 23;
  putvalue(dio,data)
  

Alternatively, you can write to individual lines through the Line property.

• putvalue(dio.Line(1:8),data)
  

To write a binary vector of values using the device object and the Line property:

• bvdata = dec2binvec(data,8);
  putvalue(dio,bvdata)
  putvalue(dio.Line(1:8),bvdata)
  

The second input argument supplied to dec2binvec specifies the number of bits used to represent the decimal value. Because the preceding commands write to all eight lines contained by dio, an eight element binary vector is required. If you do not specify the number of bits, then the minimum number of bits needed to represent the decimal value is used.

Alternatively, you can create the binary vector without using dec2binvec.

• bvdata = logical([1 1 1 0 1 0 0 0]);
  putvalue(dio,bvdata)
  

Rules for Writing Digital Values

Writing values to digital I/O lines follows these rules:

• If the DIO object contains lines from a port-configurable device, then the data acquisition engine writes to all lines associated with the port even if they are not contained by the device object.
• When writing decimal values,
• If the value is too large to be represented by the lines contained by the device object, then an error is returned.
• You can write to a maximum of 32 lines. To write to more than 32 lines, you must use a binvec value.
• When writing binvec values,
• You can write to any number of lines.
• There must be an element in the binary vector for each line you write to.
• You can always read from a line configured for output. Reading values is discussed in Reading Digital Values.
• An error is returned if you write a negative value, or if you write to a line configured for input.

Referencing Individual Hardware Lines

Reading Digital Values