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Items, Properties, and Values 121
Item generation
Property-aware blocks
Item properties include attributes, priorities, and quantities. In addition to the blocks listed above, the following blocks in the Item library provide an interface for viewing, selecting, or modifying existing item properties or for adding new ones:
Cost By Item Equation(I) History Information Query Equation(I) Queue
Queue Equation
Item generation
Queue Matching Read(I) Resource Item Select Item Out Shutdown Throw
Unbatch Workstation Write(I)
Items for a model are usually generated using the Create block. While it can also generate val- ues, the Create block can create items:
• Randomly.Arandomdistributioncausesitemstobegeneratedwitharandomorconstant interarrival time. The distribution determines the time between item arrivals; a smaller interarrival time indicates that items will arrive more frequently. See the examples below.
• Byschedule.Creatingitemsbyschedulecausesanitemtobegeneratedataspecificarrival time. The schedule defines when the item will arrive and the time between arrivals is fixed. See the examples in “Generating items according to a schedule” on page 124.
• Infinitely.Thisprovidesaninfinitesupplyofitemsthatareavailableondemand.For instance, connecting a Create block with this behavior to a Gate block would provide an item to the Gate block each time it opens.
A Create block is set to Create items infinitely should never be connected to an infinite capacity queue, since generating an infinite supply of items would overwhelm the system.
☞ In a model, each item can represent an individual entity or a collection of individual entities. For instance, 50 items coming into a model could represent 50 people or it could represent 50 bus loads of people. How you characterize items is completely up to you.
Generating items at random intervals
The Create block can generate items that arrive to the model at random times. When set to “Create items randomly”, the Create block outputs items at random intervals; the arguments of the distribution define the interarrival time.
Example model
As you saw in the Discrete Event Tutorial on page 108, the Car Wash model is an example of using the Create block to generate items at random intervals.
☞ The Car Wash model is located in the \Examples\Tutorial\Discrete Event folder.
Discrete Event

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