Page 256 - ExtendSim User Guide
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Resources and Shifts
Closed and open systems
• Using the ExtendSim database or global arrays to track resource availability, limiting the flow of items in a model. For example, item availability would be regulated by how data in the database changes during the course of a run.
• Using block combinations to control item movement as model status changes over time, such as a Queue followed by a Gate that is connected to a Read block.
Your cleverness and knowledge of ExtendSim can probably lead to even more ideas.
Closed and open systems
As discussed on page 104, blocks that provide a finite number of resources can be part of closed or open systems. In a closed system, resources are recycled back to the originating block. In an open system, resources are not recycled but instead exit the system. Systems can also be partially closed, for example when some of the resources are recycled back and others are not.
The following model uses three Resource Item blocks to illustrate a closed system (Techni- cians), an open system (Stock), and a partially closed system (Fixtures).
Closed and Open Systems model
The model assumes that about one third of the fixtures are consumed in the process; they are restocked at periodic intervals by the Create block.
Scheduling resources
To accurately characterize the impact of resources in a simulation model it is common to model resource scheduling logic, both in terms of where and when a resource should be assigned. For example, if one resource item is required in multiple places, such as an item that could be routed to two or more Batch blocks, then scheduling logic needs to be added to the model.
There are several ways resources can be scheduled. Some methods apply to using either resource pools or resource items and some apply only to scheduling resource items.
Scheduling resource pools and resource items
As discussed in the following sections, there are two systems available to schedule resource pools and resource items:
• UsetheTR(totalresources)connectoronindividualresourceblocks,asdescribedinthefol- lowing section.
Discrete Event

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