Page 331 - ExtendSim User Guide
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Holding and pushing
Item library blocks treat their output items in one of two ways:
• Most blocks hold the item and it leaves only when another block pulls it in.
• When set to create items randomly, by schedule, or infinitely, the Create block pushes the item from the block when it is generated, regardless of whether it will be picked up by another block. The Create block has to push items out, because those items are created within the block and are arrival time related.
Avoid this pitfall
When a Create block pushes an item and it is not picked up, the item disap- pears from the simulation. Generally this would only be used in certain very specific types of models. In most situa- tions where the Create block is set to create items randomly, by schedule, or infinitely, follow the block with a queue to collect the items and hold them, so that all the items generated are available for the rest of the model.
Unwise: Wise:
Tips and Techniques 305
Moving items through the simulation
Of course, if the Activity has an infinite capacity, it is not necessary to place a queue after the Create block.
A Create block set to Create items infinitely should never be connected to an infinite capac- ity queue, since generating an infinite supply of items would overwhelm the system.
Pulling and viewing
There are two ways a block’s item input connector can have access to an item: it can pull an item from the preceding block (as most connectors do), or it can simply view an item that is waiting at the item output of the preceding block. If an item input connector pulls an item in, it has access to the item for processing. However, if an item input connector only views items, it does not have direct access to them, it can only sense their presence at the preceding output connector.
The particular connectors that only view items (not pull them) are:
• TheGateblock’ssensorconnectorwhenitissettoType:areagatingoritsdemandcon-
nector when the block is set to Type: conditional gating with items.
• The item input connector on the History block, if the block has been added in parallel to another block. This is shown below and described in “Using the History block to get item information” on page 297.
Discrete Event

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