S-Function Outputs :: Writing S-Functions in M (Writing S-Functions)

Writing S-Functions

S-Function Outputs

An M-file returns an output vector containing the following elements:

sys, a generic return argument. The values returned depend on the flag value. For example, for flag = 3, sys contains the S-function outputs.

x0, the initial state values (an empty vector if there are no states in the system). x0 is ignored, except when flag = 0.

str, reserved for future use. M-file S-functions must set this to the empty matrix, [].

ts, a two-column matrix containing the sample times and offsets of the block (see Specifying Sample Time in the online documentation for information on how to specify a block's sample time and offset).

For example, if you want your S-function to run at every time step (continuous sample time), set ts to [0 0]. If you want your S-function to run at the same rate as the block to which it is connected (inherited sample time), set ts to [-1 0]. If you want it to run every 0.25 seconds (discrete sample time) starting at 0.1 seconds after the simulation start time, set ts to [0.25 0.1].

You can create S-functions that do multiple tasks, each at a different sample rate (i.e., a multirate S-function). In this case, ts should specify all the sample rates used by your S-function in ascending order by sample time. For example, suppose your S-function performs one task every 0.25 second starting from the simulation start time and another task every 1 second starting 0.1 second after the simulation start time. In this case, your S-function should set ts equal to [.25 0; 1.0 .1]. This will cause Simulink to execute the S-function at the following times: [0 0.1 0.25 0.5 0.75 1 1.1 ...]. Your S-function must decide at every sample time which task to perform at that sample time.

You can also create an S-function that performs some tasks continuously (i.e., at every time step) and others at discrete intervals.

S-Function Arguments Defining S-Function Block Characteristics