Compare Signal and Message Data Modes for Activity Diagrams

R2026b

Since R2026a

When you reference an activity diagram from a System Composer™ architecture, you can use the Data Mode property on the input parameter node to model a continuous or event-driven simulation. You can use signal mode for continuous execution from simulation start, or message mode for event-driven execution that waits for discrete data. This example shows the simulation behavior of each data mode using the same model. For background on activity diagrams, see Describe System Behavior Using Activity Diagrams. For information about input parameter nodes, see Implement Component Behavior Using Activity Diagrams.

This example uses the MessageVsSignal model. The component Generate Signal and Message has a Simulink behavior that generates the same data in form of signal and message with a delay of two seconds. The two output ports connect to different components with activity diagram behavior. One activity diagram expects signal data and the other expects message data. The activity diagrams perform a simple operation of adding 3 to the input value when a token arrives at the input parameter node.

open("MessageVsSignal.slx");

Signal Data Mode

In the signal path, the first component with Simulink behavior introduces a delay of 2 seconds before the signal reaches the activity diagram component. Because a signal is continuous, the input parameter node on the activity diagram receives a token at t = 0. The signal value is initially zero, but the token is still present. The activity begins execution immediately at simulation start. The output appears at t = 1 as the activity received a token at t = 0 and the duration of the action node is set to 1.

Message Data Mode

In the message path, the first component with Simulink behavior introduces the same delay 2 seconds before the signal reaches the activity diagram component. Unlike a signal, no message exists at the input parameter node until the component sends one. The activity diagram is idle until the first message arrives after the delay.

The activity diagram outputs a value only after the message arrives. The output is delayed relative to the start of simulation because the activity did not receive a token until the message arrived at t = 2.

Compare Behavior

Both paths have the same delay in the Simulink behavior component. The data mode setting on the input parameter node determines when the activity executes.

Behavior	Signal Mode	Message Mode
Token at input parameter node	Present from t = 0 (signal is continuous, even with value zero)	Absent until a message arrives after the delay
Activity start time	Begins at simulation start	Begins when the first message arrives
Output timing	Output appears immediately from t = 0	Output appears only after the delay
Execution pattern	Single continuous execution throughout simulation	One execution per message received