Chapter 4. CPU Operation
146
PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual
GFK-2222AD
Constant Window Mode
In Constant Window mode, each sweep can consume a variable amount of time. The Logic Window is
executed in its entirety each sweep. The CPU alternates among the three windows, allowing them to
execute for a time equal to the value set for the Constant Window timer. The overall CPU sweep time is
equal to the time required to execute the Housekeeping, Input Scan, Logic Window, and Output Scan
phases 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 Constant
Window mode.
The value of the Constant Window timer can be configured to be any value from 3 to 255 ms. The
Constant Window timer value may also be set by the programming software or by the user program
using Service Request functions #3, #4, and #5.
The following figure illustrates three successive sweeps in Constant Window mode. Note that the total
sweep times may vary due to sweep-to-sweep variations in the Logic Window, but the time given to the
Communications and Background Windows is constant. Some of the Communications or Background
Windows may be skipped, suspended, or run multiple times based on the Constant Window time.
CC
HK
INPUT
OUTPUT
CC
BPC
BG
LOGIC
SWEEP n+1
HK
INPUT
OUTPUT
CC
BPC
BG
SWEEP n
SWEEP n+2
HK
INPUT
OUTPUT
CC
BPC
LOGIC
LOGIC
CC
SYS
BG
Constant
Window
Time
Abbreviations:
HK = Housekeeping
CC = Controller Communications Window
BPC = Backplane Communications Window
BG = Background Window
Figure 43: Typical Sweeps in Constant Window Mode
4.2
Program Scheduling Modes
The CPU supports one program scheduling mode: the Ordered mode. An ordered program is executed in
its entirety once per sweep in the
Logic Window.
