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108 Tutorial
A basic discrete event model
The key to discrete event modeling is the construction of a flow diagram using blocks to repre- sent the problem’s operations and resources. The Item library is designed for building discrete event models of commercial and industrial processes. It is often used with other ExtendSim libraries, especially the Value and Plotter libraries.
The following example shows how to build a discrete event model of a car wash; it will use most of the important blocks in the Item library. Starting with a simple model, then adding complexity and features, this chapter will show how to:
• Model a single waiting line with a single server • Add a second server
• Animatethemodelin2D
• Routeitemsthroughthemodel
• Add constraining resources
• Use attributes to characterize items so they can make decisions about which route to take
This tutorial assumes you have completed the chapters in the Tutorial module that starts on page 14 and that you have read the introductory discrete event chapter that starts on page 98.
A basic discrete event model
The most common discrete event model involves the handling of one or more waiting lines or queues, such as those found in supermarkets or factories.
About the model
The Car Wash model represents a business operation where cars can be washed and waxed. The assumptions for the final model are:
• Themodelrunsforasimulatedtimeof8hours(480minutes)
• Cars arrive approximately every 4 minutes
• There is only one route into the car wash
• There are two bays, one for washing only and one for washing and waxing • It takes 6 minutes to wash a car; it takes 8 minutes to wash and wax a car
• Approximately 25% of the cars want to be waxed
• Cars have to be driven through the operation by an attendant • TheblockscomefromtheItem,Value,andPlotterlibraries
☞ The Car Wash models are located in the folder \Examples\Tutorials\Discrete Event\Car Wash. Starting a model and setting simulation parameters
The following steps are typical when starting any discrete event model.
Open a new model worksheet
Give the command Run > Simulation Setup. In the Setup tab enter the simulation parame- ters:
End time: 480
Global time units: minutes
If they aren’t already open, open the Item, Plotter, and Value libraries
Discrete Event

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