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Resources and Shifts 227
Modeling resources
order in which they were received (or in order of the requesting item’s ranking as specified in the Resource Pool dialog).
Multiple Uses model
Resource Item method
Another method for explicitly modeling resources is by using the Resource Item block. With this method, each resource is represented by an item whose purpose is to provide a service for other items in the model. The number of resources that are initially available are entered in the dialog of a Resource Item block. For an item in the model to use this type of resource, the item must be batched with the resource (see also “Batching and Unbatching” on page 207.) While the resource is batched with an item, it cannot be used elsewhere in the model. If a resource is not available, the batch will not be able to be completed, and the item will have to wait until a resource becomes available. As with the resource pool method, the movement of items in the model is restrained based on the availability or lack of resources.
If you are modeling a closed system, the resource must be unbatched from the item when it is no longer being used. Once it is unbatched, it should be routed back to the resource-type block so that it may used again. In an open system, for example where the resource is a consumable product, the resource can stay batched with the item. Closed and open systems are discussed on page 230.
Advantages and disadvantages of using resource items
A resource item can have properties such as attributes, a priority, and a quantity like any other item. For this reason, this method of modeling resources is preferred if you need to track infor- mation about resources.
☞ See “Items, Properties, and Values” on page 119 for a complete explanation of attributes and other item properties.
Some limitations of using resource items are:
• The Resource Item block must be connected in the model and the connection must be such that the resources it outputs can be batched with the items that require them.
• Theresourceitemcannot“see”theitemswaitingforit.Youmustuseroutingblocksto direct the resource item to the correct Batch or Unbatch block.
☞ After learning about resources, see “Advanced Resource Management” on page 223 for a more rigorous method of creating resource requirements, scheduling, and tracking resources.
Discrete Event

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