Page 127 - ExtendSim User Guide
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Introduction 101
Terminology and architecture
blocks. Start with an existing discrete event block as a base: either use a copy of an Item library block similar to the one you want to build, or use one of the discrete event template blocks in the ModL Tips library; those blocks’ names start with “MYO” for “Make Your Own”. The ModL code of an MYO block is commented to explain how certain features are implemented. Read the Developer Reference before modifying discrete event blocks so you have a better understanding of how those blocks work internally.
Terminology and architecture
Before building a discrete event model, it is helpful to understand the terminology that will be used and to have an overview of ExtendSim discrete event architecture.
Overview of a discrete event model
Discrete event models pass entities (called items) from block to block as events occur during the simulation run. The items in the simulation are usually generated as a random distribution within specific parameters, or as a scheduled list of when events will occur. These items can have properties, such as attributes and priorities, which help them correspond more closely to parts, customers, jobs, and so forth in real life. Items are processed by activities, and the time and extent of processing is often dependent on the availability of resources.
The main source of discrete event blocks is the Item library. Most of the blocks in the Item library have item connectors and value connectors. An item connector passes an item and all the information associated with it to the next item connector. Value connectors and dialog parameters provide specific information about the item and its properties (attributes, timing, and so on) as well as information about the effects that the item has in the model (such as queue length and wait).
☞ It is this value information which is plotted and displayed in a discrete event model, not the items themselves.
Often the object of the simulation is to determine where there are bottlenecks in the process and to see which parts of the process might be improved. Each branch of the flow diagram should either feed into another block or end in an Exit block.
A model can combine continuous blocks, typically those in the Value library, with discrete event blocks from the Item library. If you use any discrete event blocks in a model, the model will become discrete event and will require the Executive block (Item library).
Layout of a discrete event model
You can place the blocks in a model anywhere you want, remembering that ExtendSim evalu- ates discrete event blocks along the path of the connections. The only exception to this general- ity is that the Executive block (which is required for all discrete event simulations) must be to the left of all other blocks.
Executive block
The Executive block controls and does event scheduling for discrete event and discrete rate models. An Executive block must be placed to the left of all other blocks in a discrete event or discrete rate model. Its use in a model changes the timing so that simulation time advances from one event to the next, rather than at uniform intervals.
For more information, see “Executive block” on page 313.
☞ Most of the Executive’s options are for advanced users. Unless you use string attributes, it is rare that you would need to make any changes in the Executive’s dialog.
Discrete Event

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