GE PACSystems RX7i Reference Manual page 153

Chapter 4. CPU Operation
Major Phases in a Typical CPU Sweep
Phase Activity
Housekeeping The housekeeping portion of the sweep performs the tasks necessary to prepare for the start
of the sweep. This includes updating %S bits, determining timer update values, determining
the mode of the sweep (Stop or Run), and polling of expansion racks.
Expansion racks are polled to determine if power has just been applied to an expansion rack.
Once an expansion rack is recognized, then configuration of that rack and all of its modules
are processed in the Controller Communications Window.
Input Scan During the input scan, the CPU reads input data from the Genius Bus Controllers and input
modules. If data has been received on an EGD page, the CPU copies the data for that page
from the Ethernet interface to the appropriate reference memory. For details, see
PACSystems RX7i & RX3i TCP/IP Ethernet Communications User Manual, GFK-2224.
Note:
Application The CPU solves the application program logic. It always starts with the first instruction in the
Program Task program. It ends when the last instruction is executed. Solving the logic creates a new set of
Execution output data.
(Logic Window)
For details on controlling the execution of programs, refer to PACSystems RX7i, RX3i and
RSTi-EP CPU Programmer's Reference Manual, GFK-2950 Chapter 2.
Interrupt driven logic can execute during any phase of the sweep. For details, refer to
PACSystems RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual, GFK-2950 Chapter
2.
A list of execution times for instructions can be found in Appendix A.
Output Scan The CPU writes output data to bus controllers and output modules. The user program
checksum is computed.
During the output scan, the CPU sends output data to the Genius Bus Controllers and output
modules. If the producer period of an EGD page has expired, the CPU copies the data for that
page from the appropriate reference memory to the Ethernet interface. The output scan is
completed when all output data has been sent.
If the CPU is in RUN Mode and it is configured to perform a background checksum calculation,
the background checksum is performed at the end of the output scan. The default setting for
number of words to checksum each sweep is 16. If the words to checksum each sweep is set
to zero, this processing is skipped. The background checksum helps ensure the integrity of
the user logic while the CPU is in RUN Mode.
The output scan is not performed if a program has an active Suspend I/O function on the
current sweep.
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The input scan is not performed if a program has an active Suspend I/O function on
the previous sweep.
PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD