How can I create a Unit Delay block that accepts variable-size signals and does not need to be placed inside a conditionally executed subsystem in Simulink 7.7 (R2011a)?

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MathWorks Support Team
MathWorks Support Team on 23 May 2011
Edited: MathWorks Support Team on 2 Mar 2020
I have a model containing an Embedded MATLAB block which is part of an algebraic loop. This loop is broken by a Unit Delay. I now want to change the output signal of my Embedded MATLAB block to a variable-size signal. This led to the following error:
Embedded MATLAB Interface Error: Block 'Unit Delay' contains states that require resetting whenever the input signal sizes vary. This block property is inconsistent with a block property of 'Embedded MATLAB Function/ SFunction '; the output signal sizes of block 'Embedded MATLAB Function/ SFunction ' depend on its input signal values. Consider placing 'Unit Delay' in an Enabled/Function-Call/Action subsystem and setting the parameter 'Propagate sizes of variable-size signals' on the corresponding port block to: 'Only when enabling' for an Enabled or a Function-Call subsystem; or 'Only when execution is resumed' for an Action subsystem.
I followed the advice and placed the Unit Delay in an Enabled Subsystem. Since the Enabled Subsystem is atomic, the Unit Delay does not break the algebraic loop anymore. I now receive the following error: ERROR: Algebraic loop error with 'Enabled Subsystem'
I would like to introduce a delay to break an algebraic loop composed of variable-size signals.

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Accepted Answer

MathWorks Support Team
MathWorks Support Team on 2 Mar 2020
Edited: MathWorks Support Team on 2 Mar 2020
As mentioned in the error message above, the Unit delay block propagates sizes only during initialization of conditionally executed subsystems. This behavior is explained here:
http://www.mathworks.com/help/releases/R2011a/toolbox/simulink/ug/br65qom-1.html
In order to work around this situation, it is possible to implement a different Unit Delay using an S-function.
Attached to this solution is both a Level 2 MATLAB file S-Function and a C S-function which implement a Unit Delay that does need to be placed in a conditionally executed subsystem.
Note that the attached examples illustrate one possible way to implement a Unit Delay. Depending on your specific application you may want to alter what exactly happens when the size of the variable-size signal changes to obtain the behavior you are looking for.
  1 Comment
Sandip Kumar
Sandip Kumar on 5 Dec 2013
Edited: MathWorks Support Team on 11 Jun 2018
Hi Kevin,
Sorry for the delay in responding. Unfortunately, we do not provide TLC for this C code. I would recommend you create your own TLC based on examples from the Documentation and if you need suggestions/debugging can contact our technical support by creating a service request at the following link with all the information you have :
https://www.mathworks.com/support/servicerequests/create.html

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