User`s guide

Introduction

1-9

S-Function Concepts

Understanding these key concepts should enable you to build S-functions

correctly:

• Direct feedthrough

• Dynamica lly sized inputs

• Setting sample ti m es and offset s

Direct Feedthrough

Direct f eedthrough means that the output o r the variable sample time is

controlled dir ect ly by the value of an input port. A good rule of thumb is that

an S-function input port has direct f eedthr ough if:

• The output function (

mdlOutputs or flag==3) is a function of the input u.

That is, there is direct feedthroughif the input

u isused inequationsdefined

mdlOutputs. Out puts ma y a lso include graphical outputs, as in the case of

an XY Graph scope.

• It is a variable sample time S-function (calls

mdlGetTimeOfNextVarHit or

flag==4

) and the computation of the next sample hit requires the input u.

An example o f a s ystem that requires its inputs ( i.e ., has direct fe edthrough) is

the operati on , w h ere u is the input, k is the gain, and y is the out p u t.

An example of a system that does not require its inputs (i.e., does not have

direct feedthro ugh) i s t his simple integrat io n algorithm

Outputs:

Derivative:

wherex isthe state, is the state der iv ativ e withrespect to t ime,u istheinput

and y is the output. No te that is the variable that Simulink integrates. It is

veryimportant to setthe direct feedthroughflag correctlyb ecause itaffects the

executionorderof theblocksin yourmodelandis usedtodetect algebraicloops.

Dynamically Sized Inputs

S-functions can be written to support arbitrary input widths. In this case, the

actual input width is determined dynamically when a simulation is started by

evaluating the width of the input vector driving the S-function. The input

yku

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