Cycle-Time Rules

This section uses graphics to present the rules of cycle-time in an easily accessible form. The following figure shows how the software handles multiple time-stamps within a conserved-flow chain.

If you have a model that uses cross-level flows, the time-stamped material is maintained across the transition between modules.

Internal Consideration

Whenever a flow is time-stamped in a model, the software will automatically shift from the "Normal" to the "Cycle-time" Run mode. The change in the Run mode is required for the software to collect cycle-time information. The Run mode options can be found in the Time Specs dialog, under the Run menu.

Internally, the following changes occur as a result of shifting the Run mode from Normal to Cycle-time.

• The model will be set to run using Euler's integration method.
• Inflows to stocks will be "lined up" within the stocks, and will exit via outflows on a first in, first out basis. In this respect, stocks will act like queues.
• Non-negativity will be turned "on" for all stocks, and won't be accessible within the stock dialog.

Practically speaking, you should only notice two behavioral changes when the Run mode shifts to Cycle-time. First, it's possible that the execution of a simulation run will be somewhat slower than it might be otherwise. This is because of the additional calculational overload that's brought about by the generation of cycle-time metrics. Second, any stocks that have taken on negative values in the Normal mode (because Non-negativity has been turned "off" in their dialogs) will be kept from taking on negative values.

It's always possible for you to override the shift from Normal to Cycle-time Run mode. To do so, just enter the Time Specs dialog and select the Normal option. If you do shift to the Normal Run mode, cycle-time calculations will be disabled. As a result, cycle-time builtins will return a value of zero. All time-stamped flows will lose the clock face on their icons. Their check boxes will be turned "off". Similarly, all cycle-time converters will lose the clock face on their icons.

You can shift back to Cycle-time mode by selecting the Cycle-time option within the Time Specs dialog. Or, you can time-stamp any flow in your model. When you shift back to Cycle-time mode, the software will remember all of your previous work with cycle-time. The clock face will return on the icons for all the flows that were previously time-stamped, and for all the converters that contain cycle-time builtins.