UDP out - Ethernet UDP output mailbox

SD019 Posted on April 2, 2021 Updated on November 25, 2025 Download PDF pdf icon

Table of Contents

This page is based on SDK 2025.2.

The UDP output mailbox block allows sending up to 1024 bytes of data via Ethernet using the UDP/IP protocol (in SDK versions prior to 2025.2, the size limit is 8 bytes). To receive data, the UDP input mailbox should be used.

The data to send via UDP is formatted as a vector. The Signal decoding format parameter defines the data type of each element, and the Number of signals parameter defines the length of the output vector. The data can be interpreted as one of the following types: int8, int16, int32, uint8, uint16, uint32, float32, or float64.

Example: if Signal decoding format is set to uint32 (4 bytes) and Number of signals is 100, then:

  • Size of the UDP data = 4 × 100 = 400 bytes
  • Output = vector of 100 uint32 elements

The block supports two Data transmission mode:

  • On-demand: the user manually triggers the message transmissions.
  • Periodically: the message is sent periodically, whether the data has been changed or not. The user can configure the transmission frequency.

Simulink block

Signal specification

  • The first input is the data to send. The accepted data type is specified by the Signal coding format parameter. This data can be provided as a vector, with its size defined by the Number of signals parameter.
  • The second input is the send data signal. It is used to initiate a data transmission when the Data transmission mode is set to On-demand mode has been selected. Data is sent upon a rising edge on this signal.

Parameters

  • Ethernet port number: sets the destination port to which the data will be sent.
  • Ethernet IP address : selects the IP address to whom the data will be sent.
  • Signal coding format: defines the data type accepted in the data input (int8, int16, int32, uint8, uint16, uint32, float32, or float64).
  • Number of signals: specifies the vector size of the data to be sent.
  • Byte order : defines the byte order in which the data will be sent. (Little-endian or Big-endian)
  • Tx frequency : sets the data transmission frequency if the Data transmission mode is set to Periodically.
Ethernet output mailbox Simulink dialog parameters port configuration
Ethernet output mailbox Simulink dialog parameters IP address

PLECS block

  • The first input is the data to send. The accepted data type is specified by the Signal coding format parameter. This data can be provided as a vector, with its size defined by the Number of signals parameter.
  • The second input is the send data signal. It is used to initiate a data transmission when the Data transmission mode is set to On-demand mode has been selected. Data is sent upon a rising edge on this signal.

Parameters

  • Ethernet port number: sets the destination port to which the data will be sent.
  • Ethernet IP address : selects the IP address to whom the data will be sent.
  • Signal coding format: defines the data type accepted in the data input (int8, int16, int32, uint8, uint16, uint32, float32, or float64).
  • Number of signals: specifies the vector size of the data to be sent.
  • Byte order : defines the byte order in which the data will be sent. (Little-endian or Big-endian)
  • Tx frequency : sets the data transmission frequency if the Data transmission mode is set to Periodically.
Ethernet output mailbox PLECS dialog

C++ functions

bool Eth_ConfigureOutputMailbox(unsigned int mailboxId, unsigned int port, const char* ipAddress, float maxTxFrequency = 100, tEndianness endianness = LITTLE_ENDIAN, size_t msgLength = 4);Code language: C++ (cpp)

Configures an Ethernet UDP output mailbox.

It has to be called in UserInit().

Parameters

  • mailboxId:  a unique id used to distinguish mailboxes from each other. This id must be unique throughout the code for all ETH and CAN input/output mailboxes.
  • port: the destination UDP port number to which the data will be sent.
  • ipAddress: the destination IP address to whom the data will be sent.
  • maxTxFrequency: maximal frequency at which data can be sent. The frequency must be a multiple of the main interrupt frequency. If the requested frequency is not achievable, it will automatically be set to the closest valid frequency.
  • endianness: defines the bytes order. BIG_ENDIAN (most significant byte first) or LITTLE_ENDIAN (least significant byte first)
  • msgLength: size in bytes of the message, for best perfomance it is advised to keep it under 1024 bytes

Return value

  • bool: returns false if too many output mailboxes were created or if the port was already assigned to another ETH output mailbox.
int Eth_Write(unsigned int mailboxId, void* data, size_t size);Code language: C++ (cpp)
int Eth_Write(unsigned int mailboxId, unsigned int dataLow);Code language: C++ (cpp)
int Eth_Write(unsigned int mailboxId, int dataLow);Code language: C++ (cpp)
int Eth_Write(unsigned int mailboxId, float dataLow);Code language: C++ (cpp)

These functions are used to send data on Ethernet using UDP.

They have to be called during the control interrupt.

Parameters

  • maildboxId: a unique ID used to distinguish mailboxes from each other. This ID must be unique throughout the code for all ETH and CAN input/output mailboxes.
  • data: data which will be sent.
  • size: size in bytes of the data to copy, must match the size declared during mailbox creation

Return value

  • int: returns 1 if the data has been loaded in the write buffer. Returns 0 otherwise.

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