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ExtendSim moves items in a discrete event model only when an event happens. Events are occurrences such as receipt of an order, a telephone call, or a customer arriving. They are man- aged by the Executive block (discussed on page 313) and only occur when particular blocks specify that they should.
Blocks that depend on time cause events to happen at the appropriate time. For instance, an Activity block holding an item until a particular time will cause an event to be posted to the ExtendSim internal event calendar. When the time is reached, the event occurs and the model recalculates its data.
Blocks that do not generate events allow the blocks after them to pull items during a single event. Thus a single event can cause an item to pass through many blocks if those blocks do not stop them. For instance, a Set block could set the item’s attribute and pass the item to the next block in the same event.
For more information, see “Event scheduling” on page 316.
Activities are undertaken to achieve a specified outcome, typically either a product or a ser- vice. They have a duration and usually involve the use of process elements and resources. An activity could involve processing, moving, transporting, or otherwise manipulating an item. For more information, see the chapter “Processing” on page 177.
Resources are the means by which process activities and operations are performed. Typical resources include equipment, personnel, space, energy, time, and money. Resources can be available in unlimited quantities but are most often limited or constrained. They can be con- sumed in the process or kept for reuse.
In ExtendSim, a resource that is required for a process or activity to take place can be modeled either as an item or as a unit in a pool:
• Itemresourcesgetbatchedwiththeitemsthatrequirethem(ResourceItemmethod).
• Aresourcepoolcontainsacountoftheresourcesthatareavailabletothemodel.Thecount can be a number in a Resource Pool block (Resource Pool method) or the number of records in an internal ExtendSim database (Advanced Resource Management method).
See “Resources and Shifts” on page 221 for complete information.
Most of the discrete event blocks pass an item index through item connectors at each event. Each passed index contains a set of information about the item – its attributes, priority, quan- tity, and so on. This is different from value connectors which only pass values.
Blocks in the Item library can contain value connectors as well as item connectors. When com- bining discrete event blocks with blocks from other libraries, you will only be able to connect compatible connectors. Item connectors can only connect to item or universal connectors; they cannot connect with value input or output connectors. Likewise, value connectors can only connect with value or universal connectors; they cannot connect with the item input or output connectors. For more information about connector types, see “Connector types” on page 574.
Introduction 103
Terminology and architecture
Discrete Event

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