User manual MATLAB SIMEVENTS 3

[. . . ] SimEvents® 3 User's Guide How to Contact The MathWorks Web Newsgroup www. mathworks. com/contact_TS. html Technical Support www. mathworks. com comp. soft-sys. matlab suggest@mathworks. com bugs@mathworks. com doc@mathworks. com service@mathworks. com info@mathworks. com Product enhancement suggestions Bug reports Documentation error reports Order status, license renewals, passcodes Sales, pricing, and general information 508-647-7000 (Phone) 508-647-7001 (Fax) The MathWorks, Inc. 3 Apple Hill Drive Natick, MA 01760-2098 For contact information about worldwide offices, see the MathWorks Web site. SimEvents® User's Guide © COPYRIGHT 2005­2010 by The MathWorks, Inc. The software described in this document is furnished under a license agreement. The software may be used or copied only under the terms of the license agreement. [. . . ] To learn more, see "Overview of Simultaneous Events" on page 3-2. 4 If the updated value of the p signal differs from its previous value, the Output Switch block selects a new entity output port. Ensuring that the port selection is up-to-date before the entity departs from the switch is the reason for the Store entity before switching option. See "Using the Storage Option to Prevent Latency Problems" on page 6-2 for details. 5 The entity departs from the selected entity output port of the Output Switch block. Top-Level Model 10-20 Examples Using Discrete Event Subsystem Blocks Subsystem Contents Example: Ending the Simulation Upon an Event This example ends the simulation as described in "Timing for the End of the Simulation" on page 10-2, precisely when the number of entities in a queue first equals or exceeds a threshold. In the next model, the Compare To Constant and Stop Simulation blocks are in a discrete event subsystem. The Discrete Event Inport block inside the subsystem has Type of signal-based event set to Sample time hit so that the subsystem executes at exactly those times when the FIFO Queue block updates the value of the queue length signal. Top-Level Model 10-21 10 Controlling Timing with Subsystems Subsystem Contents Example: Sending Unrepeated Data to the MATLAB Workspace Suppose you want to log statistics to the MATLAB workspace, but you want to save simulation time and memory by capturing only values that are relevant to you. You might want to suppress repeated values, for example, or capture only values that represent an increase from the previous value. In the next model, the discrete event subsystem contains a To Workspace block whose Save format parameter is set to Structure With Time. The Discrete Event Inport block inside the subsystem has Type of signal-based event set to Change in signal and Type of change in signal value set to Either, so that the MATLAB workspace variable tells you when the Output Switch block selects an entity output port that differs from the previously selected one. If, for example, the switch is configured to select the first port that is not blocked, then a change in the port selection indicates a change in the state of the simulation (that is, a previously blocked port has become unblocked, or a port becomes blocked that previously was not). Top-Level Model 10-22 Examples Using Discrete Event Subsystem Blocks Subsystem Contents Example: Focusing on Events, Not Values This example counts the number of times a signal changes its value, ignoring times at which the signal might be updated with the same value. The discrete event subsystem contains a Counter Free-Running block with an inherited sample time. Because the Counter Free-Running counts starting from 0, the subsystem also adds 1 to the counter output. The Discrete Event Inport block inside the subsystem has Type of signal-based event set to Change in signal and Type of change in signal value set to Either, so that the subsystem is executed each time the input signal changes its value. In contrast to other subsystem examples, this subsystem does not use the signal's specific values for computations; the input signal is connected to a Terminator block inside the subsystem. The counter's value is what the subsystem sends to the MATLAB workspace. In this example, avoiding extraneous calls to the subsystem is not merely a time-saver or memory-saver, but rather a strategy for producing the correct result. 10-23 10 Controlling Timing with Subsystems Top-Level Model Subsystem Contents Example: Detecting Changes from Empty to Nonempty This example executes a subsystem only when an N-server changes from empty to nonempty, or vice versa, but not when the number of entities in the server remains constant or changes between two nonzero values. In the model, the N-Server block produces a #n signal that indicates the number of entities in the server. The server is empty if and only if the #n signal is 0. Connected to the #n signal is a Discrete Event Inport block inside the subsystem that has Type of signal-based event set to Trigger and Trigger type set to Either, so that the subsystem is executed each time the #n signal changes from 0 to 1 or from 1 to 0. Connected to the w signal is another Discrete Event Inport block inside the subsystem; this block has Execute 10-24 Examples Using Discrete Event Subsystem Blocks subsystem upon signal-based events cleared so that this signal does not cause additional calls to the subsystem; the subsystem merely uses the most recent value of the w signal whenever the #n signal exhibits a trigger edge. Note Because the N-Server block updates the w signal before updating the #n signal, both signals are up to date when the trigger edge occurs. If the server changes instantaneously from empty to nonempty and back to empty, the subsystem is called exactly twice in the same time instant, once for the rising edge and once for the subsequent falling edge. The Triggered Subsystem block might not detect the edges that the zero-duration value of 1 creates, and thus might not call the subsystem at that time instant. This is why the Discrete Event Subsystem block is more appropriate for this application. Example: Logging Data About the First Entity on a Path Suppose your model includes a particular entity path that entities rarely use, and you want to record certain attributes of the first entity that takes that path during the simulation. [. . . ] This example uses a Counter Free-Running block inside a subsystem to count the number of times the subsystem is called. (Be aware that the Counter Free-Running block starts counting from zero, not one. ) The discrete-event portion of the simulation involves a D/D/1 queuing system in which the server is never idle for a nonzero period of time. As a result, the #n signal exhibits many zero-duration values, shown in the plot below. 16-32 Zero-Duration Values and Time-Based Blocks The example uses two approaches to try to call the subsystem each time the server's #n signal rises from 0 to 1: · The approach using a Triggered Subsystem is unsuitable because it does not count changes that occur in a time interval of length zero. You can see from the Display block that the triggered subsystem is never called. [. . . ]