Writing your own block with discrete states (MATLAB S-Function) 5

Posted by Guy Rouleau,

A few weeks ago, I noticed the following question on MATLAB Answers by K E:

What are continuous and discrete states in Simulink?

In my opinion, the best way to understand how states work in Simulink is by implementing an S-Function. So this week we will see how to create a MATLAB S-Function with discrete states.

If you are not familiar with MATLAB S-Functions, I recommend looking at my previous post introducing the basics of MATLAB S-Functions

What is a state?

Before starting to implement our S-Function, let's see the answer my friend Kaustubha Govind provided to the above question:

In my own words, a crude word to use in place of "state" is "memory". A state adds memory to a system in such a way that the output at a given time depends not only on its current input, but also on its previous inputs. There is a more formal explanation about states here.

Discrete states can be thought purely as internal memory - for example a Unit Delay block has one discrete state...

Let's stop here and see how to implement a Unit Delay in a MATLAB S-Function.


In the setup function, we define one parameter, one input port and one output port.

Direct Feedthrough?

"Does this block have Direct Feedthrough?" This is the question Simulink needs to know to determine the sorting order for all the blocks in the model. Direct feedthrough means that the Outputs function uses the input variable in its calculation. Since we are not planning to use the input value in the Outputs function to calculate the output value, we can set the DirectFeedthrough property of the input port to false.

Finally, we need to register 4 methods: PostPropagationSetup, InitializeConditions, Outputs and Update.

Here is how this looks:

setup function of a Unit Delay

Post Propagation Setup

In terms of S-Function, the discrete state is stored in what we call a work vector, the Dwork. The properties of the work vector are defined in the DoPostPropSetup callback method.

In our case, we need one Dwork vector, of dimension 1, and we want to mark it as a discrete state:

Declaring a Dwork vector

Setting the UsedAsDiscState to true makes the states visible to the Simulink engine. That way it will be included if we save the model states and will be seen by Simulink.BlockDiagram.getInitialState. If you prefer to hide the internal state of your block, you can set UsedAsDiscState to false.

Initial Conditions

When the simulation starts, you can set the initial value of the state. Here we do it using a dialog parameter

Initializing the work vector


Now the simulation loop can start. First, the Outputs method of the block is called. We read the value stored in the work vector and assign it to the output port:

Output method of the unit delay S-Function


After the Outputs method is completed, we grab the input and store it in the work vector. This is done in the Update method:

Update method of the unit delay S-Function

The next time the Outputs method runs, this value will be available.


It is important to realize here that Simulink does nothing to the discrete states. The solver only stores the values to be used in the next step.

Next week, we will follow a similar process with continuous states.

Now it's your turn

Share your S-Function experiences by leaving a comment here.

5 CommentsOldest to Newest

Vladica Sark replied on : 2 of 5

Great explanation in just a few steps. I was reading huge manuals, and found everything I need in 2 pages.

mani kandan replied on : 3 of 5

Hi Iam facing an error while evaluating s-function’s input as complex variable. I want to know whether Matlab level-1 S-function accepts complex data as input??

@mani: I never tried complex input signal for a level-1 MATLAB S-Function. But I can tell you that Level-1 s-function are very limited and I would not be surprised if it does not support complex input. I strongly recommend moving to Level-2 MATLAB S-function.

Kunchala Anil replied on : 5 of 5

whats the difference in using Output and update with and without Sampling time.
with Sampling time we can know how often the update and output will execute by using ( total simulation time / sampling time offset value) = no of time steps

if we do not assign sampletime [block.SampleTimes] value then how often the update will execute.

how can we find the time of each update block execution

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