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100 Introduction
Blocks for building discrete event models
• Processes are organized around events, such as the receipt of parts, a request for service, or the ringing of a telephone. Events occur at random but somewhat predictable intervals and can be economic or noneconomic. Events are what drive most businesses.
Industrial and commercial processes therefore represent the utilization and underactivity of elements and resources driven by events.
Blocks for building discrete event models
The blocks in the Item library are used to build discrete event models. In addition, third-party developers have created customized discrete event libraries. Plus, you can program customized discrete event blocks.
Item library
Blocks in the Item library correspond to typical activities, operations, and resources in many environments. These blocks are connected in an activity or data flow diagram that represents a system. The complexities of generat- ing and posting events are handled within the blocks, alleviating the need to do any programming in the ModL language.
Item library blocks are optimized for modeling service, manufacturing, material handling, transportation, and other discrete systems. They incorpo- rate high-level modeling concepts such as variable batching, conditional routing, and preemptive operations as well as blocks that represent machines, labor, conveyors, and so forth. Built-in performance calculations and statistical reports allow you to predict the value, effectiveness, and cost of implementing changes before committing resources.
These blocks have been specifically designed to meet most discrete event modeling needs, allowing you to quickly and easily perform complex mod- eling tasks. For instance, you can use a popup menu in a Queue block to specify that stored items are sorted in first-in-first-out order, last-in-first-out order, or in a custom order based on their assigned priorities or attributes.
As mentioned in the Tutorial, blocks from the Value library are frequently
used for data management and model-specific tasks in discrete event mod-
els. Using Value library blocks with Item library blocks does not change the fundamental architecture of discrete event models; they will still be event-based rather than use the time- based architecture of continuous models.
☞ See “Item Library Blocks” on page 851 and “Value Library Blocks” on page 843 for a listing and brief description of the blocks in those libraries.
Third-party libraries
Third-party developers use the ExtendSim environment to create libraries of blocks custom- ized for specific fields, such as semiconductor manufacturing or multi-stage manufacturing systems. For more information about products that can be purchased from third party vendors, please go to
Creating custom discrete event blocks
Because of the Item library’s extensive capability, it is not likely that you would need to pro- gram your own discrete event blocks. If you do want to do this, it is important to note that dis- crete event blocks use different data structures and programming methods than continuous
Activity Batch
Catch Item Convey Item Cost By Item Cost Stats Create Equation (I) Executive Exit
History Information Queue Queue Equation
Item library blocks
Discrete Event

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