Schedule Multiple Subsystems in a Single Step

This example shows how to design a ladder logic scheduler in Stateflow®. The ladder logic scheduler design pattern allows you to specify the order in which multiple Simulink® subsystems execute in a single time step. Stateflow schedulers extend control of subsystem execution in a Simulink model, which determines order of execution implicitly based on block connectivity and sample time propagation.

Key Behavior of the Ladder Logic Scheduler

In this example, the Ladder Logic Scheduler chart broadcasts a series of function-call output events to execute three function-call subsystems (A1, A2, and A3). During each time step:

1. The Simulink model activates the Edge to Function chart at the rising edge of the 1-millisecond pulse generator.

2. The Edge to Function chart broadcasts the function-call output event call to activate Ladder Logic Scheduler chart.

3. The Ladder Logic Scheduler chart uses sequencing ladder logic to broadcast function-call output events based on the values of the input signals u1 and u2.

The chart evaluates each condition sequentially. When a condition is valid, the chart calls the send operator to broadcast an output event. The corresponding subsystem computes its output and returns control back to the Ladder Logic Scheduler chart.

Run the Ladder Logic Scheduler

When you simulate the model, the scope shows the input and output of each function-call subsystem.

During each time step, the Ladder Logic Scheduler chart executes the subsystems depending on the values of the input signals u1 and u2:

1. If u1 is positive, the chart sends a function-call output event to execute subsystem A1. This subsystem multiplies the value of u1 by a gain of 3 and passes this value back to the Ladder Logic Scheduler chart as input u2. Control returns to the next condition in the Ladder Logic Scheduler chart.

2. If u2 is greater than 1, the chart sends a function-call output event to execute subsystem A2. This subsystem decreases the value of u2 by 1. Control returns to the final condition in the Ladder Logic Scheduler chart.

3. If u2 is less than 2, the chart sends a function-call output event to execute subsystem A3. This subsystem multiplies its input by a gain of 2.

In the scope, horizontal segments indicate time steps when a subsystem does not execute.