GE VersaMax PLC User Manual page 95

Housekeeping includes the tasks necessary to prepare for the start of the sweep. Before starting the
actual sweep, the CPU:
calculates the sweep time
Schedules the start of the next sweep
Determines the mode of the next sweep
Start of Sweep
Updates the fault reference tables
Housekeeping
Resets the Watchdog timer
If the PLC is in Constant Sweep Time mode, the sweep is delayed until the required sweep time
elapses. If the required time has already elapsed, the OV_SWP %SA0002 contact is set, and the
sweep continues without delay. Next, the CPU updates timer values (hundredths, tenths, and
seconds).
When the sweep starts, the CPU first scans inputs from input modules and option modules that
provide input-type data. Modules are scanned in ascending reference address order. Discrete input
modules are scanned before analog input modules. The CPU stores this new input data in the
appropriate memories. If the CPU has been configured to not scan I/O in Stop mode, the input scan
Input Scan
is skipped when the CPU is in Stop mode.
For CPUE05, if the CPU is in run mode and the consumer period of an Ethernet Global Data
exchange has expired, the CPU copies the data for that exchange from the Ethernet interface to the
appropriate reference memory.
Next, the CPU solves the application program logic. It always starts with the first instruction in the
Application Program
program. It ends when the END instruction is executed. Solving the logic creates a new set of output
Logic Scan
data.
Immediately after the logic solution, the CPU scans all output modules in ascending reference
address order. The output scan is completed when all output data has been sent.
If the CPU has been configured to not scan I/O in Stop mode, the output scan is also skipped when
Output Scan
the CPU is in Stop mode.
For CPUE05, if I/O is enabled and the producer period of an Ethernet Global Data exchange has
expired, the CPU copies the data for that exchange from the appropriate reference memory to the
Ethernet interface.
If there is a programming device attached, the CPU next executes the programmer communications
window. The programmer communications window will not execute if there is no programmer
Programmer
attached.
Communications
In the default limited window mode, each sweep the CPU honors one service request. The time limit
Window
for programmer communications is 6 milliseconds. If the programmer makes a request that requires
more than 6 milliseconds to process, the processing is spread out over multiple sweeps.
Next, the CPU processes communications requests from intelligent option modules. The modules
are polled in roundrobin fashion, so no module has priority.
In default ("Run to Completion") mode, the length of the system communications window is limited to
System
400 milliseconds. If a module makes a request that requires more than 400 milliseconds to process,
Communications
the request is spread out over multiple sweeps.
Window
In Limited mode, option modules that communicate with the PLC using the system window have less
impact on sweep time, but response to their requests is slower.
CPU Operation
For public disclosure
Parts of the CPU Sweep
GFK-1503E User Manual 95