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CAPL Scripting Tutorial For Automotive Engineers

CAPL (Communication Access Programming Language) is a scripting language specifically developed for use with Vector’s CANoe and CANalyzer tools. It provides a structured way to simulate and test automotive communication networks, making it a crucial tool for engineers working with Controller Area Network (CAN), Local Interconnect Network (LIN), FlexRay, and Ethernet protocols.

The primary goal of CAPL is to allow users to define event-driven behaviors, automate network interactions, and analyze communication data. It enables users to create test cases, simulate Electronic Control Units (ECUs), and inject faults into a network, ensuring the robustness of automotive systems.

CAPL scripts operate based on event-driven programming, meaning actions are executed when specific events occur on the network. These events include message reception, timer triggers, and user inputs, making CAPL an essential tool for real-time testing and automation.

With CAPL scripting, automotive engineers can replicate real-world scenarios efficiently, reducing the need for physical ECUs and hardware components. This enhances testing efficiency, lowers costs, and speeds up the development cycle of vehicle communication networks.

Why Use CAPL Scripting?

CAPL scripting offers multiple advantages in the development and validation of automotive communication systems. One of its biggest strengths is its ability to automate testing, enabling engineers to simulate CAN and LIN network messages without requiring actual vehicle components. This feature is particularly useful in hardware-in-the-loop (HIL) and software-in-the-loop (SIL) testing.

Another major benefit of CAPL is its support for custom event handling. Engineers can define specific behaviors for messages, timers, and user inputs, allowing for precise control over test scenarios. This flexibility ensures that various real-world conditions can be replicated with ease.

CAPL is also widely used in network gateway development, where messages from different communication protocols need to be converted and transmitted between ECUs. This makes it a valuable tool in designing advanced driver-assistance systems (ADAS) and connected vehicle technologies.

Additionally, CAPL helps improve efficiency in debugging and analysis. Engineers can log and analyze network behavior using CAPL scripts, identifying issues in communication flow, timing, and signal integrity. This capability significantly reduces development time and enhances system reliability.

CAPL Script Structure

CAPL scripts follow a structured format consisting of includes, global variables, event procedures, and user-defined functions. These elements work together to enable effective automation and simulation of network behaviors.

01. Includes and Defines

Includes and defines are used to specify constants, libraries, and macros that will be utilized in the script. This helps improve code organization and maintainability.

#define ENGINE_RPM 1000  // Defining a constant

02. Global Variables

Global variables store persistent data that is used throughout the script. These variables are accessible across multiple event procedures and functions.

int counter; // Global variable to store count

03. Event Procedures

CAPL scripts are based on event-driven programming, meaning different event handlers execute code when a specific event occurs on the CAN bus.

a. on start()

The on start() event is triggered when the CAPL script is executed. It is commonly used to initialize variables, set timers, or send startup messages.

on start {
   write("CAPL Script Started");
}

b. on message

The on message event is triggered when a specific message is received on the network. It is useful for monitoring and responding to incoming data.

on message CAN1.EngineData {
   write("Engine Data Received: %d", this.RPM);
}

c. on timer

Timers in CAPL are used for the periodic execution of functions. This is useful for generating cyclic messages or implementing delays.

timer myTimer;

on start {
   setTimer(myTimer, 1000);  // Set a timer for 1 second
}

on timer myTimer {
   write("Timer Triggered");
   setTimer(myTimer, 1000);  // Restart the timer
}

d. User-Defined Functions

CAPL allows users to create custom functions to enhance script modularity. These functions help avoid code repetition and improve maintainability.

void sendMessage(int rpm) {
   message CAN1.EngineData msg;
   msg.RPM = rpm;
   output(msg);
}

Coding-Based CAPL Script Interview Questions

01. Write a basic CAPL script and explain the structure.

Code:

variables {
  int counter;  // Global variable
}

on start {
  counter = 0;
  write("CAPL Program Initialized");
}

Explanation:

variables {} section declares global variables.
on start {} is an event that triggers when the simulation starts.

02. Write a CAPL script to send a CAN message.

Code:

message 0x100 msgExample; // Declare CAN message with ID 0x100

on start {
  msgExample.dlc = 8;  // Set Data Length Code (DLC)
  msgExample.byte(0) = 0x11;
  msgExample.byte(1) = 0x22;
  output(msgExample);  // Send message
}

Explanation:

The message msgExample with ID 0x100 is defined.
DLC (Data Length Code) is set to 8 bytes.
Bytes are assigned values.
output(msgExample); sends the message.

03. Write a CAPL script to receive a CAN message and process its data.

Code:

on message 0x200 {
  write("Received Message: ID=0x200, Data= %02X %02X", this.byte(0), this.byte(1));
}

Explanation:

The event on message 0x200 triggers when a message with ID 0x200 is received.
this.byte(n) extracts data bytes from the received message.

04. Write a CAPL script to execute a function every 100ms using a timer.

Code:

variables {
  timer myTimer;
}

on start {
  setTimer(myTimer, 100);
}

on timer myTimer {
  write("Timer Event Triggered");
  setTimer(myTimer, 100);
}

Explanation:

setTimer(myTimer, 100); starts the timer.
on timer myTimer {} executes every 100ms.

05. Write a CAPL script to log CAN messages to a file.

Code:

variables {
  dword fileHandle;
}

on start {
  fileHandle = fopen("log.txt", "w");
  if (fileHandle == 0) write("File opening failed!");
}

on message * {
  fprintf(fileHandle, "Msg ID: 0x%X Data: %02X %02X %02X\n", this.ID, this.byte(0), this.byte(1), this.byte(2));
}

on stop {
  fclose(fileHandle);
}

Explanation:

fopen(“log.txt”, “w”); opens a file for writing.
on message * logs all received CAN messages.
fclose(fileHandle); closes the file when the simulation stops.

06. Write a CAPL function to calculate the sum of two numbers.

Code:

int add(int a, int b) {
  return a + b;
}

on start {
  int result = add(5, 10);
  write("Sum = %d", result);
}

Explanation:

int add(int a, int b) defines a function.
result = add(5, 10); calls the function.

07. Write a CAPL script to execute code when a key is pressed.

Code:

on key 'A' {
  write("Key A Pressed");
}

Explanation:

The on key ‘A’ event triggers when the ‘A’ key is pressed.

08. Write a CAPL script to check if a received message has a specific byte value.

Code:

on message 0x300 {
  if (this.byte(0) == 0xAA) {
    write("Special Byte Received!");
  }
}

Explanation:

if (this.byte(0) == 0xAA) checks if the first byte is 0xAA.

09. Write a CAPL script using a `for` loop.

Code:

on start {
  int i;
  for (i = 0; i < 5; i++) {
    write("Loop iteration %d", i);
  }
}

Explanation:

A for loop runs 5 times, printing the iteration number.

10. Write a CAPL script using an array.

Code:

variables {
  int dataArray[5];
}

on start {
  dataArray[0] = 10;
  dataArray[1] = 20;
  write("First Value: %d, Second Value: %d", dataArray[0], dataArray[1]);
}

Explanation:

dataArray[5] defines an array of 5 integers.
Values are stored and accessed.

11. Write a CAPL script to send a CAN message every 500ms.

Code:

variables {
  message 0x400 msgCyclic;
  timer cyclicTimer;
}

on start {
  setTimer(cyclicTimer, 500);  // Start the cyclic timer
}

on timer cyclicTimer {
  msgCyclic.dlc = 2;
  msgCyclic.byte(0) = 0xAB;
  msgCyclic.byte(1) = 0xCD;
  output(msgCyclic);
  setTimer(cyclicTimer, 500);  // Restart the timer
}

Explanation:

A timer is used to send a CAN message every 500ms.
The message 0x400 is sent with 2 bytes of data.

12. Write a CAPL script to process only messages with IDs 0x500 and 0x600.

Code:

on message * {
  if (this.ID == 0x500 || this.ID == 0x600) {
    write("Filtered Message ID: 0x%X", this.ID);
  }
}

Explanation:

if (this.ID == 0x500 || this.ID == 0x600) filters messages with specific IDs.

13. Write a CAPL script using a `switch-case` to process different message IDs.

Code:

on message * {
  switch (this.ID) {
    case 0x700:
      write("Message ID 0x700 received");
      break;
    case 0x701:
      write("Message ID 0x701 received");
      break;
    default:
      write("Unknown message ID: 0x%X", this.ID);
  }
}

Explanation:

switch (this.ID) allows processing different message IDs efficiently.

14. Write a CAPL script to check if a specific bit is set in a message byte.

Code:

on message 0x800 {
  if (this.byte(0) & 0x01) {
    write("Bit 0 is set in byte 0");
  }
}

Explanation:

this.byte(0) & 0x01 checks if bit 0 of the first byte is set.

15. Write a CAPL script using a structure for CAN messages.

Code:

typedef struct {
  int id;
  byte data[8];
} CanMessage;

variables {
  CanMessage msg;
}

on start {
  msg.id = 0x900;
  msg.data[0] = 0x11;
  msg.data[1] = 0x22;
  write("Struct Message ID: 0x%X, Data[0]: %02X", msg.id, msg.data[0]);
}

Explanation:

typedef struct defines a custom CAN message structure.

16. Write a CAPL script to log events to a file.

Code:

variables {
  dword logFile;
}

on start {
  logFile = fopen("event_log.txt", "w");
  if (logFile == 0) write("Error opening log file!");
}

on message * {
  fprintf(logFile, "Received Message ID: 0x%X\n", this.ID);
}

on stop {
  fclose(logFile);
}

Explanation:

fopen(“event_log.txt”, “w”) opens a log file for writing.
fprintf(logFile, …); logs received message IDs.

17. Write a CAPL script to detect bus-off and restart CAN communication.

Code:

on busOff {
  write("Bus Off detected! Restarting CAN...");
  restartCAN();
}

Explanation:

The on busOff {} event detects bus-off and calls restartCAN();.

18. Write a CAPL script to calculate and display the CAN bus load.

Code:

variables {
  float busLoad;
}

on timer 1s {
  busLoad = getBusLoad();
  write("Current CAN Bus Load: %f%%", busLoad);
  setTimer(1s, 1000);
}

Explanation:

getBusLoad(); retrieves the current CAN bus load in percentage.

19. Write a CAPL script to extract and process a signal from a CAN message.

Code:

on message 0xA00 {
  int rpm = (this.byte(1) << 8) | this.byte(0);
  write("Engine RPM: %d", rpm);
}

Explanation:

The RPM signal is extracted using bitwise operations from 2 bytes.

20. Write a CAPL script to handle CAN error frames.

Code:

on errorFrame {
  write("Error Frame Detected! Error Type: %d", this.errorType);
}

Explanation:

The on errorFrame {} event triggers when a CAN error occurs.
this.errorType provides the error type.

CAPL Scripting in Real-World Applications

CAPL scripting is widely used in various automotive testing and validation processes. One of its most common applications is automated testing, where engineers use CAPL scripts to send, receive, and validate network messages without requiring actual vehicle hardware.

Another key use case is fault injection testing, where CAPL is used to introduce errors into network communication. This helps in assessing the robustness of automotive systems and identifying potential failure points.

Logging and data analysis is another significant area where CAPL is beneficial. Engineers use CAPL scripts to capture network traffic, analyze communication patterns, and identify anomalies in data transmission.

CAPL is also used in gateway development, where messages from different protocols (e.g., CAN to LIN) need to be converted and transmitted. This is crucial in vehicle network architectures where multiple ECUs need to communicate seamlessly.

Best Practices for CAPL Scripting

To maximize efficiency and maintainability, it is essential to follow best practices when writing CAPL scripts. One important practice is to use meaningful variable names, which enhances code readability and debugging.

Avoiding global variables where possible is another recommended practice. Instead, use function parameters and local variables to ensure better data encapsulation.

Keeping scripts modular by defining reusable functions helps in maintaining clean and structured code. This approach makes debugging and extending the script much easier.

Lastly, using write() statements for debugging during development can help in monitoring the execution flow and identifying potential issues early in the testing process.

Conclusion

CAPL scripting is a powerful tool for automotive network simulation and testing. By leveraging CAPL, engineers can automate complex testing scenarios, analyze network behavior, and ensure the robustness of vehicle communication systems. Mastering CAPL can significantly improve efficiency in CAN, LIN, and other automotive communication protocols.

Further Learning

Vector’s official CAPL documentation
Hands-on practice with CANoe/CANalyzer
CAPL scripting forums and online communities

This was about “CAPL Scripting Tutorial For Automotive Engineers“. Thank you for reading.

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About The Author

Chetan Shidling

Automotive | Technical Blogger | Engineer | Content Creator

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CAPL Scripting Tutorial For Automotive Engineers

Why Use CAPL Scripting?

CAPL Script Structure

01. Includes and Defines

02. Global Variables

03. Event Procedures

Coding-Based CAPL Script Interview Questions

01. Write a basic CAPL script and explain the structure.

02. Write a CAPL script to send a CAN message.

03. Write a CAPL script to receive a CAN message and process its data.

04. Write a CAPL script to execute a function every 100ms using a timer.

05. Write a CAPL script to log CAN messages to a file.

06. Write a CAPL function to calculate the sum of two numbers.

07. Write a CAPL script to execute code when a key is pressed.

08. Write a CAPL script to check if a received message has a specific byte value.

09. Write a CAPL script using a for loop.

10. Write a CAPL script using an array.

11. Write a CAPL script to send a CAN message every 500ms.

12. Write a CAPL script to process only messages with IDs 0x500 and 0x600.

13. Write a CAPL script using a switch-case to process different message IDs.

14. Write a CAPL script to check if a specific bit is set in a message byte.

15. Write a CAPL script using a structure for CAN messages.

16. Write a CAPL script to log events to a file.

17. Write a CAPL script to detect bus-off and restart CAN communication.

18. Write a CAPL script to calculate and display the CAN bus load.

19. Write a CAPL script to extract and process a signal from a CAN message.

20. Write a CAPL script to handle CAN error frames.

CAPL Scripting in Real-World Applications

Best Practices for CAPL Scripting

Conclusion

Further Learning

About The Author

Chetan Shidling

09. Write a CAPL script using a `for` loop.

13. Write a CAPL script using a `switch-case` to process different message IDs.