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196 Processing
Interrupting processing
the shutdown event, and the block will keep items, resume process after shutdown.
☞ With this information and the settings in the Activity’s Shutdown tab, processing will shut down for any item that is already in the Activity at time 2 and that item will not resume pro- cessing until time 4.5. Likewise, any item in the Activity at time 7 will be shut down and not resume processing until time 8, and so forth.
When shutdown is triggered by an item connection to SD, the selected items being processed by the Activity block will be shut down at the time and for the duration specified. However, because the block supports parallel processing, if items arrive to the block after the shutdown has been triggered, those items will be processed normally. For complete shutdown of the block, use a value connection instead of an item connection.
When the model is run, the plotter will show both the number of items processed (obtained from the Exit block) and the timing and duration of the shutdowns (obtained from the SD out- put on the Activity.) The SD output gives a value of 1 while an item is shutdown and a value of 0 while it is not.
The Shutdown block
The Shutdown block provides for dependent or independent failure policies and can be used to model the failure or scheduled downtime for a single or multiple components.
A shutdown event can either be random (e.g. a failure) or intentional (e.g. scheduled mainte- nance). The Shutdown block manages both types of events and broadcasts the overall down status as either a value or item signal. Typically these signals are used to directly control either the Activity (Item library) or the Valve (Rate library).
• Random failures are modeled on the Shutdown block’s first tab where distributions can be selected for the Time-Between-Failure (TBF) and Time-To-Repair (TTR).
• Scheduled shutdowns are modeled either by connecting to the Shutdown’s “sched” input or by going to the block’s Options tab and linking the Shutdown block to a Shift block. Note that there are policy options allowing the modeler to specify the impact an intentional down event will have on random downs. For example, when a maintenance event is scheduled, should progress towards the next failure event be reset?
Whether random or scheduled, there are two types of down signals:
• Valuedownsignalsbydefaultare“1”fordownand“0”forup.However,ontheShutdown’s first tab you can control what the up and down values should be. For example, if the Shut- down block is connected to the “R” connector on the Valve block, it would make more sense to have the down = 0 in order to close the Valve during a down state.
• An item down signal is generated by the Shutdown block once for every shutdown event. Each item has two attributes that indicate to the Activity which items to shut down and for how long. Typically, this option is used when you want to target specific items to shut down inside the Activity.
When a database table is used to specify TBF and TTR distributions, one Shutdown block can be used to model any number of component failures. The components (and their respective fail behaviors) that are modeled in one particular Shutdown block are typically related in some way. For example, you would want to use one Shutdown block to model the failure of a bicy- cle’s tires, chain and brakes because all three components are only wearing out while the bike is being ridden, and if one component fails, the wearing on the other two stops. It would be
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