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Why Does Reading Data from the dsp.AsyncBuffer Object Give a Dimension Mismatch Error in the MATLAB Function Block?

If you are reading data from an asynchronous buffer inside a MATLAB Function (Simulink) block, the block throws a dimension mismatch error if the output of the read method is not specified to be a variable-size signal.

Here is the bufferWrapper function that contains the algorithm inside the MATLAB Function block. When input on the cmd port is 1, the dsp.AsyncBuffer object writes the data input, u, to the buffer. When input on the cmd port is 0, the object reads data from the buffer.

function [y,isData] = bufferWrapper(u,cmd) 

persistent asyncBuff 
if isempty(asyncBuff) 
   asyncBuff = dsp.AsyncBuffer; 
if cmd % write
    y = zeros(3,1);
    isData = false;
else % read
    y = read(asyncBuff,3);
    isData = true;

You must initialize the buffer by calling either write or setup before the first call to read.

During the write operation, the first output, y, is zeros(3,1) and the second output, isData, is 0. During the read operation, y is the data in the buffer and isData is 1.

Run the model and the following error occurs.

The output of read(asyncBuff,3) on line 14 is variable sized. The output is variable sized because the size of the signal output by the read function depends on the input arguments to read. To resolve this error, specify y as a variable-size signal.

  1. In the MATLAB Function block Editor, click Edit Data to open the Ports and Data Manager.

  2. For the output y, select the Variable size check box.

  3. Click Apply.

Run the model and view the output y in the Time Scope.

With cmd = 0, no data is written into the buffer. Therefore, the output is 0. To write the input data u to the buffer, set cmd = 1. After you write some data, if you change cmd back to 0, the Time Scope output changes to the following.

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