Message Link

Introduction

The message_link class lets two V5 Brains communicate by sending short string messages with optional small data. It’s designed for robot-to-robot coordination — like starting routines, sharing sensor results, or triggering behaviors on another robot.

Every usage of send adds a message to the linked V5 Brain’s queue, and the queue is read first-in, first-out (FIFO). If multiple messages are sent before the other V5 Brain uses receive, they will be stored and returned one at a time in the order they were sent. Because messages can queue, repeatedly sending the same status every loop may create backlog; for time-critical logic, send only when values change.

Important: Both robots must be running projects that use message_link at the same time, or no messages will be sent/received.

Class Constructors

message_link( 
  int32_t     index, 
  const char  *name, 
  linkType    type, 
  bool        isWired = false );

Class Destructor

Destroys the message_link object and releases associated resources.

virtual ~message_link();

Parameters

Parameter	Type	Description
index	int32_t	The Smart Port that the V5 Radio, or the Smart Cable, is connected to, written as PORTx, where x is the port number (for example, PORT1).
name	char	The name of the Message Link.
type	linkType	The type of link, either linkType::manager or linkType::worker. This information is used to correctly configure the radio and also determines available bandwidth for transmission and reception. A manager robot has double the available bandwidth (1040 bytes/second) to send information to the worker robot (520 bytes/second). linkType::manager - The message_link object is a manager. linkType::worker - The message_link object is a worker.
isWired	bool	Whether the message_link object is wired. true - The message_link object is wired. false - The message_link object is wireless.

Example

Code for Robot 1

// Create a wireless link in Port 1
message_link link = message_link(
  PORT1, // index
  "VEXRoboticsLink123456789", // name
  linkType::manager, // type
  true); // isWired

Code for Robot 2

// Create a wireless link in Port 1
message_link link = message_link(
  PORT1, // index
  "VEXRoboticsLink123456789", // name
  linkType::worker, // type
  true); // isWired

Member Functions

The message_link class includes the following member functions:

• isLinked — Returns whether this Brain is actively connected to its paired Brain.

• send — Sends a message to the paired Brain.

• receive — Waits for and returns the next incoming message.

• received — Registers a function to be called whenever a new message is received.

Before calling any message_link member functions, a message_link instance must be created, as shown below:

Code for Robot 1

// Create a wireless link in Port 1
message_link link = message_link(
  PORT1, // index
  "VEXRoboticsLink123456789", // name
  linkType::manager, // type
  true); // isWired

Code for Robot 2

// Create a wireless link in Port 1
message_link link = message_link(
  PORT1, // index
  "VEXRoboticsLink123456789", // name
  linkType::worker, // type
  true); // isWired

isLinked

isLinked method returns whether the V5 Brains on a MessageLink are paired with one another.

Available Functions

bool isLinked();

Parameters

This function does not have any parameters.

Return Values

This function returns a boolean value.

• true – The two V5 Brains are paired and communicating on this link.

• false – The two V5 Brains are not paired on this link.

Notes

It is good practice to always check to ensure the V5 Brains are linked at the start of a project before running any further code.

Examples

Code for Robot 1

// Tell the other robot when the screen is being pressed
  // Create the link
  message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::manager, true);

  // Do not run code UNTIL the Brains are linked
  while (!link.isLinked()){
    wait(0.1, seconds);
  }

  Brain.Screen.print("Robot 1 - Manager");

  // Continuously send if the screen is being pressed
  while (true){
    if (Brain.Screen.pressing()){
      link.send("pressed");
    } else {
      link.send("released");
    }
    wait(0.05, seconds);
  }

Code for Robot 2

// Display if the other robot's screen is being pressed

  // Create the link
  message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::worker, true);
  
  // Do not run code UNTIL the Brains are linked
  while (!link.isLinked()){
   wait(0.1, seconds);
  }

  // Continuously check if Robot 1 has sent "pressed"
  while (true){
    Brain.Screen.clearScreen();
    Brain.Screen.setCursor(1, 1);
    Brain.Screen.print("Robot 2 - Worker");
    Brain.Screen.newLine();
    char msg[32] = {0};                         // buffer for incoming message
    int32_t n = link.receive(msg, sizeof(msg), 100);  // wait up to 100ms
    if (n > 0 && strcmp(msg, "pressed") == 0) {
      Brain.Screen.newLine();
      Brain.Screen.print("Manager is being pressed!");
    }
    wait(0.05, seconds);
  }

send

Sends a message over the link connection.

Available Functions

1 — Sends a message string.

int32_t send( 
  const char *message );

2 — Sends a message string with a double.

int32_t send( 
  const char *message, 
  double      value );

3 — Sends a message string with an int32_t and a double.

int32_t send(
  const char *message, 
  int32_t     index, 
  double      value );

Parameters

Parameter	Type	Description
message	const char	The message to send to the other linked V5 Brain.
index	int32_t	An integer to send to the other linked V5 Brain.
value	double	A double to send to the other linked V5 Brain. Doubles are formatted to 4 decimal places; if fewer decimals are provided, trailing zeros are added until 4 are shown.

Return Values

This function does not return any values.

Notes

If only message is sent, the other linked V5 Brain receives that exact message.

If value (and index if provided) are included, the receiving Brain gets a single encoded message that combines all fields in this order: .message_index_value

Examples

Using link.send(“string”, 1, 2.55) is received as .string_1_2.5500.

Code for Robot 1

// Tell the other robot when the screen is being pressed

// Create the link
message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::manager, true);

// Do not run code UNTIL the Brains are linked
while (!link.isLinked()){
  wait(0.1, seconds);
}

Brain.Screen.print("Robot 1 - Manager");

// Continuously send if the screen is being pressed
while (true){
  if (Brain.Screen.pressing()){
    link.send("pressed");
  } else {
    link.send("released");
  }
  wait(0.05, seconds);
}

Code for Robot 2

// Display if the other robot's screen is being pressed

// Create the link
message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::worker, true);

// Do not run code UNTIL the Brains are linked
while (!link.isLinked()){
  wait(0.1, seconds);
}
// Continuously check if Robot 1 has sent "pressed"
while (true){
  Brain.Screen.clearScreen();
  Brain.Screen.setCursor(1, 1);
  Brain.Screen.print("Robot 2 - Worker");
  Brain.Screen.newLine();
  char msg[32] = {0};  // buffer for incoming message
  int32_t n = link.receive(msg, sizeof(msg), 100);  // wait up to 100ms
  if (n > 0 && strcmp(msg, "pressed") == 0) {
    Brain.Screen.newLine();
    Brain.Screen.print("Manager is being pressed!");
  }
  wait(0.05, seconds);
}

receive

Receives a message from the link connection.

Available Functions

int32_t receive(
  char     *buffer,
  int32_t   length, 
  int32_t   timeoutMs = 500 );

Parameters

Parameter	Type	Description
buffer	char	The buffer to store the received message.
length	int32_t	The length of the buffer.
timeoutMs	int32_t	How long in milliseconds receive will wait for a new message only if the queue is empty. Default is 300,000 (300 seconds).

Return Values

This function returns the number of bytes received.

Notes

• receive returns the next queued message from the other linked V5 Brain. Messages are read in FIFO order (oldest unread first). If the queue is empty when receive is called, it waits up to the specified timeout for a new message. If no message arrives in that window, receive returns 0, and any message sent afterward remains in the queue to be read the next time receive is used.

• If the other linked V5 Brain sent only a string, receive returns that exact string.

• If the other linked V5 Brain sent an integer and/or float, receive returns a single encoded string that combines all fields in this order: .string_integer_float

  • For example, if the other linked V5 Brain uses link.send(“string”, 1, 2.55), then using link.receive() returns an encoded string such as .string_1_2.5500. You can split this encoded string (see the examples below) to extract the message name, integer, and float values.

  • If you’d rather avoid manual splitting/parsing, use received to register a handler for a specific message name; the handler receives the sent values as arguments.

Examples

Using link.send(“string”, 1, 2.55) is received as .string_1_2.5500.

Code for Robot 1

// Tell the other robot when the screen is being pressed

// Create the link
message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::manager, true);

// Do not run code UNTIL the Brains are linked
while (!link.isLinked()){
  wait(0.1, seconds);
}

Brain.Screen.print("Robot 1 - Manager");

// Continuously send if the screen is being pressed
while (true){
  if (Brain.Screen.pressing()){
    link.send("pressed");
  } else {
    link.send("released");
  }
  wait(0.05, seconds);
}

Code for Robot 2

// Display if the other robot's screen is being pressed

// Create the link
message_link link = message_link(PORT1, "VEXRoboticsLink123456789", linkType::worker, true);

// Do not run code UNTIL the Brains are linked
while (!link.isLinked()){
  wait(0.1, seconds);
}
// Continuously check if Robot 1 has sent "pressed"
while (true){
  Brain.Screen.clearScreen();
  Brain.Screen.setCursor(1, 1);
  Brain.Screen.print("Robot 2 - Worker");
  Brain.Screen.newLine();
  char msg[32] = {0};   // buffer for incoming message
  int32_t n = link.receive(msg, sizeof(msg), 100);  // wait up to 100ms
  if (n > 0 && strcmp(msg, "pressed") == 0) {
    Brain.Screen.newLine();
    Brain.Screen.print("Manager is being pressed!");
  }
  wait(0.05, seconds);
}

received

Registers a function to be called whenever the V5 Brain receives a sent message.

Available Functions

1 Registers a callback for all received messages (message + value).

void received(
    void (* callback)(const char *, const char *, double)
);

2 Registers a callback for all received messages (message + index + value).

void received(
    void (* callback)(const char *, const char *, int32_t, double)
);

3 Registers a callback for a specific message (message + value).

void received(
    const char *message,
    void      (* callback)(const char *, const char *, double)
);

4 Registers a callback for a specific message (message + index + value).

void received(
    const char *message,
    void      (* callback)(const char *, const char *, int32_t, double)
);

Parameters

Parameter	Type	Description
message	const char *	The message name to listen for. If omitted, the callback runs for all received messages.
callback	Function pointer	A function that executes when a message is received.

Return Values

This function does not return any values.

Callback Signature

1 Receives sender name, message name, and value.

void callback(const char *, const char *, double)

2 Receives sender name, message name, index, and value.

void callback(const char *, const char *, int32_t, double)

Type	Description
1st const char *	The sender link name.
2nd const char *	The message name received.
int32_t	The index associated with the message.
double	The value associated with the message.

Examples

Code for Robot 1

// Send whether screen is pressed on left or right
void screen_pressed() {
  if (Brain.Screen.xPosition() < 240) {
    link.send("left");
  } else {
    link.send("right");
  }
}

int main() {
  /* vexcodeInit() is only required when using VEXcode.
  Remove vexcodeInit() if compiling in VS Code. */
  vexcodeInit();
  
// Create the link
  message_link link(PORT1, "LeftRightLink", linkType::manager, true);

// Do not run code UNTIL the Brains are linked
  while (!link.isLinked()) {
    wait(0.1, seconds);
  }

  Brain.Screen.print("Robot 1 - Manager");

  Brain.Screen.pressed(screen_pressed);

  while (true) {
    wait(10, msec);
  }
}

Code for Robot 2

// Create variable to track where screen is pressed
int lastPress = 0;

// Update the variable whenever a new message is received
void message_received(const char* message, const char* linkname, double timestamp) {
  if (strcmp(message, "left") == 0)  lastPress = 1;
  if (strcmp(message, "right") == 0) lastPress = 2;
}

int main() {
  /* vexcodeInit() is only required when using VEXcode.
  Remove vexcodeInit() if compiling in VS Code. */
  vexcodeInit();

// Create the link
  message_link link(PORT1, "VEXRoboticsLink123456789", linkType::worker, true);

// Do not run code UNTIL the Brains are linked
  while (!link.isLinked()) {
    wait(0.1, seconds);
  }

  link.received(message_received);

  // Display if Robot 1's screen was pressed on the left or right
  while (true) {
    Brain.Screen.clearScreen();
    Brain.Screen.setCursor(1, 1);
    Brain.Screen.print("Robot 2 - Worker");
    Brain.Screen.newLine();

    if (lastPress == 1) {
      Brain.Screen.print("Manager pressed LEFT");
    } else if (lastPress == 2) {
      Brain.Screen.print("Manager pressed RIGHT");
    } else {
      Brain.Screen.print("Waiting...");
    }

    wait(0.1, seconds);
  }
}