Standard Cpu Sweep Operation; The Sweep Windows; The Watchdog Timer; Constant Sweep Time Operation - GE VersaMax PLC User Manual

7.2 Standard CPU Sweep Operation

CPU Operation
For public disclosure
Standard Sweep operation is the normal operating mode of the PLC CPU. In Standard
Sweep operation, the CPU repeatedly executes the application program, updates I/O, and
performs communications and other tasks shown in the diagram:
1. The CPU performs its start-of-sweep housekeeping tasks.
It reads inputs.
2.
3. It executes the application program.
4. It updates outputs.
5. If a programming device is present, the CPU communicates with it.
6. It communicates with other devices.
It performs diagnostics.
7.
Except for communicating with a programmer, all these steps execute every sweep.
Programmer communications occur only when needed.
In this mode, the CPU performs all parts of its sweep normally. Each sweep executes as
quickly as possible with a different amount of time consumed each sweep.

7.2.1 The Sweep Windows

The programmer communications window and the system communications window have
two operating modes:
Limited Mode
Run to Completion
Mode
SVCREQ 2 can be used in the application program to obtain the current times for each
window.

7.2.2 The Watchdog Timer

When the CPU is in Standard Sweep mode, the Watchdog Timer catches failure
conditions that could cause an unusually long sweep. The length of the Watchdog Timer
is 500 milliseconds. It restarts from zero at the beginning of each sweep.
If the sweep takes longer than 500mS, the OK LED on the CPU module goes off. The
CPU resets, executes its powerup logic, generates a watchdog failure fault, and goes to
Stop mode. Communications are temporarily interrupted.

7.2.3 Constant Sweep Time Operation

If the application requires that each CPU sweep take the same amount of time, the CPU
can be configured to operate in Constant Sweep Time mode. This operating mode assures
that the inputs and outputs in the system are updated at constant intervals. This mode can
also be used to implement a longer sweep time, to assure that inputs have time to settle
after receiving output data from the program.
The execution time of the window is 6ms. The window
terminates when it has no more tasks to complete or when
6ms has elapsed.
Regardless of the time assigned to a particular window the
window runs until all tasks within that window are completed
(up to 400ms).
GFK-1503E User Manual 97