Receive — Control Systems/Messages And Events

Receive

Control Systems / Messages And Events

Reads messages: on each of its sample hits it takes one message from the message line into its input and outputs the value v it carries. A message taken is consumed. On a hit with no message waiting, v holds the value of the last message taken – or reads the Initial value, by the Value source when queue is empty – and before the first message it reads the Initial value either way.

Ports

• Input msg – a message line, from a Send or another block that sends messages; its messages are any size [m,n], or a bus (a Hit Crossing Probe’s).
• Output status – only with Show queue status On, and then the first output, as in Simulink: a boolean, 1 on a hit that took a message and 0 on one that found none.
• Output v – the value, the size of the messages; the second output when the status is shown. When the messages are a bus, v is that bus (bus), element for element, for a Bus Selector to read.

Parameters

• Use internal queue – Off (default): the block keeps one message, and a newer one overwrites it, so a reader slower than its sender reads the newest. On: it keeps a queue, as the next three set.
• Queue overwriting – with the queue on: On (default) drops the oldest message when a new one arrives at a full queue; Off drops the new one, and the run reports how many were dropped, as R2026a warns.
• Queue length – with the queue on: how many messages it keeps, a whole number of at least 1. Default 1.
• Queue type – with the queue on, which message is taken first:
  • LIFO (default) – the newest.
  • FIFO – the oldest.
  • Priority – refused: it orders messages by a field of a bus message, which ICore’s queues do not read.
• Priority order – Ascending (default) or Descending; read only by a Priority queue, so it has no effect here.
• Show queue status – Off (default): one output, v. On: two, the status then v. The output count must agree: add or remove the port to match, or the run is refused with the block’s name.
• Initial value – what v reads before the first message: a scalar, broadcast, or a matrix the size of the messages. [], the default, reads 0. With bus messages only 0 or [], which zero every element; any other value is refused, as R2026a refuses it.
• Value source when queue is empty – Hold last value (default) or Use initial value.
• Sampling Time (s) – zero or less inherits the solver's rate; a positive value runs the block at that period.

Code export

Python, MATLAB, Java, Rust, C and C++: the message line is one fixed-capacity queue in the generated core, held beside the signals, and it keeps its reader’s policy (a Receive’s queue, or a message-triggered subsystem’s). Each call pops the line’s queue: a message replaces the value held and sets the status, and with none the value holds or reads the Initial value, as live. A bus message rides the queue as its elements’ numbers in order, and v is the bus record built from them.

VHDL, Verilog, SystemVerilog and PLC Structured Text refuse a message line by name: a hardware queue is left until a hardware design asks for one. A message line that crosses the boundary of the subsystem being exported is refused by every target.

Simulink bridge

Import and export, mapped to simulink/Messages & Events/Receive. Use internal queue ↔ UseInternalQueue, Queue overwriting ↔ QueueOverwriting and Show queue status ↔ ShowQueueStatus (Off/On ↔ off/on), Queue length ↔ QueueLength, Queue type ↔ QueueType and Priority order ↔ PriorityOrder (one value for one value), Initial value ↔ InitialValue, Value source when queue is empty ↔ ValueSourceWhenQueueIsEmpty, and "Sampling Time (s)" ↔ SampleTime, as on every block. Simulink moves the status port itself.

Notes

• Stateful: it keeps the messages waiting and the last value taken.
• Measured on R2026a: a Hit Crossing Probe into a Receive gives its HitCrossMessage bus, with [] or 0 as the Initial value; -9 is refused (BusParameterMustBeZeroOrStruct).
• Measured on R2026a: a Send every 0.25 s into a Receive every 1 s with no queue reads 100, 101, 102 (the newest); with a FIFO queue of 3, 100, 100.5, 101.5, 102.5; with a FIFO queue of 2 and overwriting off, 100, 100.25, 100.5, 101.25, 102.25, and 11 messages dropped.