Introduction to the C-Script Block
1 Introduction
The C-Script block is a versatile tool in the PLECS component library that can be used for implement-
ing custom controllers and components. The advanced capabilities of the C programming language
combined with the flexible sample time settings in the C-Script block allow almost any custom com-
ponent model, from a simple mathematical function to a complex state machine, to be implemented.
The C-Script block can also simplify the workflow when writing C code for DSP controllers since the
code can be reused for the DSP. The key skills that you will learn in this exercise are:
• Understand how the C-Script block interfaces with the simulation engine through function calls.
• Understand the different time settings available in the C-Script block.
• Use the C-Script block for implementing a mathematical function.
• Use the C-Script block for implementing a discrete and continuous PI controller.
Before you begin Ensure the file buck_converter.plecs is located in your working directory. You
should also have the reference files that you can compare with your own models at each stage of the
exercise.
2 Function Call Interface
The C-Script block interfaces with the simulation engine using a number of predefined function calls.
These function calls are depicted in Fig. 1. Each function call corresponds to a code window in the
C-Script editor, which is accessed from a pull-down menu. The most commonly used code windows
are described below, and for a complete description, one can refer to the PLECS User Manual or the
block’s documentation, which is accessed by clicking the Help button.
code declarations() In addition to the function windows, a Code declarations window is pro-
vided for defining global variables, macros and helper functions to be used in the C-Script block func-
tions. The code declarations code is essentially a global header file. All variables and functions defined
within this window are globally visible to all functions within the C-Script block.
start() The Start function code window is for initializing the simulation. Internal state variables,
for example, should be initialized here.
output() The Output function code window is designed to contain the functional code for the C-
Script block. A call is made to this function at least once during each simulation time step. For this
reason, any internal states or persistent variables should be updated in the update function.
update() If a model contains discrete internal states, a call is made to the code in the Update func-
tion code window directly after the output function has been executed. When the C-Script contains
internal states, they should be updated in this function rather than in the output function to ensure
they are updated only once during each time step.
3 Parameters
When you open the C-Script block the Setup tab of the code editor window as shown in Fig. 2 will ap-
pear. In this window you can configure the parameters. You will actually write your C code within the
function windows contained in the Code tab.
3.1 Sample time parameter
The Sample time setting is a key parameter that controls when the C-Script block is called. The sam-
ple time can be inherited from the simulation engine or controlled by the C-Script block itself. A de-
scription of the possible sample time settings is given below:
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Introduction to the C-Script Block
Calculate
outputs
Calculate
updates
Calculate
outputs
Calculate
outputs
Start
simulation
Terminate
simulation
Main
loop
Event
detection
loop
Integration
loop
Calculate
derivatives
Calculate
zero-crossings
Figure 1: Function calls made during operation of the C-Script block. The update function is called
when discrete states are defined and the derivative function is called when continuous states
are defined.
Figure 2: C-Script editor window for configuring parameters and writing code.
Continuous The continuous time setting is selected by entering 0 into the sample time dialog. With
the continuous time setting, the time steps are inherited from the solver. Every time the solver takes a
step, the C-Script block is executed.
Discrete The discrete-periodic time setting is selected by entering a positive number into the sample
time dialog. The C-Script block is executed at discrete regular intervals defined by this sample time.
Variable The variable time setting is selected by entering -2 into the sample time dialog. With the
discrete-variable time setting, the next time step is determined dynamically by the C-Script block it-
self by setting the NextSampleHit built-in macro. The NextSampleHit must be initialized at the begin-
ning of the simulation to a value greater than or equal to the CurrentTime macro. See below for more
information on macros in the C-Script block.
3.2 Other parameters
The other parameters are described completely in the C-Script block’s documentation. However, it is
worth noting the following at this stage. When creating a C-Script block that contains static variables,
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Introduction to the C-Script Block
you can add discrete states to create global static variables. The discrete states are accessed using a
macro command DiscState.
3.3 List of commonly-used macros
The C-Script block contains a number of built-in macro functions that can be used to interact with the
model or solver. Some of the commonly used macros are:
InputSignal(j, i) Reference the ith signal of the jth C-Script block input.
OutputSignal(j, i) Reference the ith signal of the jth C-Script block output.
DiscState(i) Reference a discrete state with index i.
NextSampleHit Set the next call time for the C-Script block. This variable is used
when the variable sample-time setting is active.
CurrentTime Retrieve the current simulation time.
SetErrorMessage(”msg”) Abort the simulation with an error message.
4 Exercise: Implement a Mathematical Function
In this exercise, you will use the C-Script block to implement the sine function with an offset value.
Your Task:
1 Create a new simulation model, and place a C-Script component and two Constant source blocks
into it. Label the first Constant block “Offset” and set its value to 0.5. Label the second Constant
block “Frequency” and set its value to 2π · 50. Use a Signal Multiplexer block to route the two con-
stant values into the C-Script block. Your simulation model should look like that shown in Fig. 3.
Frequency
Value:
2*pi*50
C
Value:
0.5
Offset
0.5
C-Script
C-Script
Scope
Figure 3: Implementing the function y = 0.5 + sin(2π50 · t) with the C-Script block.
2 You will then need to configure the C-Script block and write the code. To configure the C-Script
block, open the block by double-clicking and in the Setup tab set the Number of inputs to 2 and
the Number of outputs to 1. Set the Sample time setting to 0 to select a continuous, or inherited
sample time. To write the sine function you will need to use the cmath library (math.h header). In
the Code declarations window of the Code tab, enter the following code:
#include <math.h>
#define offset InputSignal(0,0)
#define freq InputSignal(0,1)
In the Output function code window, enter the following code to create the sine function:
OutputSignal(0,0) = sin(freq*CurrentTime) + offset;
3 Run the simulation: Set the simulation parameters of the PLECS solver to the following:
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