# Run Specs

Run Specs... brings forth a dialog which enables you to specify the simulation length, the time step between calculations (DT), the interval between simulation pauses, the integration method, the time unit for the model, and the run mode (Normal or Cycle-time) for the model. The Run Specs dialog is shown in Fig 3-25.

Figure 3-25 Run Specs Dialog

From: - To:

Simulation length is defined by entering numbers for "From" and "To." "To" must always be greater than "From." If DT is greater than 1, and is not evenly divisible into the length of simulation, the software will choose the closest length of simulation which is a multiple of DT.

DT:

The Run Specs dialog also affords you the opportunity to set the simulation time step (DT). DT can be set by entering the desired time step directly. You can enter any number less than or equal to one or any integer greater than one. Alternately, you can check the "DT as fraction" box. Then set the number of calculations which will occur per unit time. In either case, choose a DT which is small enough to enable you to capture the shortest time frame of interest in your model. The trade-off here is speed versus accuracy. The more calculations the computer has to make per unit of simulation time, the longer it will take to complete any given simulation. For more about DT, refer to Chapter 12.

Pause Interval:

The default setting for Pause interval is infinity, designated by the letters INF on Windows machines or the infinity symbol (option-5 on your keyboard) on Macintosh machines. When the Pause interval is set at a value less than the length of the simulation, model simulation runs will pause as you have specified. Users then can inspect results. They also can change model constants, and then resume the simulation run by choosing Run from the Run menu. Thus, your models can be set up in a gaming mode for users.

Integration Method:

The Integration Method radio buttons allow you to choose among Euler's, Runge-Kutta second-order, and Runge-Kutta fourth-order methods. Briefly, use Euler's integration method unless you anticipate that your model will generate an oscillation. If you feel an oscillation is likely, and you are not using Queue, Conveyor or Oven stock types in your model, choose either RK 2 or RK 4 (try RK 2 first). More information about the integration methods can be found in Chapter 13.

Analyze Mode:

Analyze mode uses computer RAM to store model output for ALL model variables. When analyze mode is ON, a small graph will appear when you hover over variables, when the model simulation is paused or stopped.

Unit of Time:

Within Run Specs, you can define the time unit for model simulations. Simply select one of the pre-defined time units, or select Other and type in your own.

Run Mode:

Radio buttons allow you to specify the run mode for your model as "Normal," or "Cycle-time." Cycle-time is turned on automatically whenever you time-stamp a flow in order to generate and collect cycle-time metrics. When operating in the Cycle-time mode, the integration method is set to Euler's. This setting cannot be altered, as long as Cycle-time is on. Chapter 10 details the workings of time-stamping and Cycle-time calculations.

Interaction Mode:

Interaction mode gives you a choice between the Normal mode and Flight Sim mode. If you elect Flight Sim mode, the Pause interval box will disappear, and the Pause interval will gray. The options for causing the simulation to pause, when in this mode, are to choose Pause or Stop from the Run Menu. The posting of a message will also cause the simulation to Pause, even when in Flight Sim mode.

In Flight Sim mode, when you move a Slider Input, the simulation will continue running as it incorporates your input. This contrasts with the Normal interaction mode, in which clicking on a slider will pause the simulation. Note that Flight Sim mode is not available when Sensitivity Analysis is turned on. Flight Sim will always be turned off when Sensitivity Analysis is turned on.

Sim Speed:

When Flight Sim mode is selected, the Sim Speed control will appear within the Run Specs dialog. This control allows you to slow down the speed of simulation to offer the user time to react to changes in the model. This feature cannot be used to increase the speed of simulation.