Chapter 4. CPU Operation146 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222ADConstant Window ModeIn Constant Window mode, each sweep can consume a variable amount of time. The Logic Window isexecuted in its entirety each sweep. The CPU alternates among the three windows, allowing them toexecute for a time equal to the value set for the Constant Window timer. The overall CPU sweep time isequal to the time required to execute the Housekeeping, Input Scan, Logic Window, and Output Scanphases of the sweep plus the value of the Constant Window timer. This time may vary due to sweep-to-sweep variances in the execution time of the Logic Window.An application that requires a certain amount of time between the Output Scan and the Input Scan,permitting inputs to settle after receiving output data from the program, would be ideal for ConstantWindow mode.The value of the Constant Window timer can be configured to be any value from 3 to 255 ms. TheConstant Window timer value may also be set by the programming software or by the user programusing Service Request functions #3, #4, and #5.The following figure illustrates three successive sweeps in Constant Window mode. Note that the totalsweep times may vary due to sweep-to-sweep variations in the Logic Window, but the time given to theCommunications and Background Windows is constant. Some of the Communications or BackgroundWindows may be skipped, suspended, or run multiple times based on the Constant Window time.CCHKINPUTOUTPUTCCBPCBGLOGICSWEEP n+1HKINPUTOUTPUTCCBPCBGSWEEP n SWEEP n+2HKINPUTOUTPUTCCBPCLOGIC LOGICCCSYSBGConstantWindowTimeAbbreviations:HK = HousekeepingCC = Controller Communications WindowBPC = Backplane Communications WindowBG = Background WindowFigure 43: Typical Sweeps in Constant Window Mode4.2 Program Scheduling ModesThe CPU supports one program scheduling mode: the Ordered mode. An ordered program is executed inits entirety once per sweep in the Logic Window.