Page 355 - Programmable Logic Controllers, Fifth Edition - Mobile version

is already open and its contents are visible. Clicking on
Controller RSLogix the files in the folder. The 2 sign indicates that the folder
Controller Tags the right mouse button brings up many different, context-
Controller Fault Handler sensitive popup menus. Often, you find that this is a short-
Power-Up Handler cut to access the property window or menu options from
Tasks the menu bar.
Continuous
Program_01
Program Tags Tasks
MainRoutine
Periodic Tasks are the first level of scheduling within a project. A
Program_02 task is a collection of scheduled programs. When a task
Program Tags is executed, the associated programs are executed in the
MainRoutine order listed. This list of programs is known as the program
Unscheduled Programs schedule. Tasks provide scheduling based on specific con-
HMI ditions and do not contain any executable code. Only one
Program Tags task may be executing at any given time. The number of
Main
Motion Groups tasks a controller can support depends on the specific con-
Trends troller. The main types of tasks (Figure 15-7) include:
Data Types • Continuous tasks execute nonstop but are always
User-Defined
Strings interrupted by a periodic task. Continuous tasks
Predefined have the lowest priority. A ControlLogix continuous
Module-Defined task is similar to the File 2 in the SLC 500 platform.
I/O Configuration Here the continuous task is named Main Task.
• Periodic tasks function as timed interrupts. They
Figure 15-6 Controller organizer tree. interrupt the continuous task and execute for a fixed
length of time at specific time intervals.
• Event tasks also function as interrupts. Rather than
The RSLogix 5000 controller organizer (Figure 15-6) being an interrupt on a timed basis, an event task
displays the project organization in a tree format showing is triggered by an event that happened or failed to
tasks, programs, routines, data types, trends, I/O configu- happen.
ration and tags. Each folder groups common functions
together. This structure simplifies the navigation and the Programs
overall view of the whole project.
In front of each folder, there is an icon containing a Programs are the second level of scheduling within a
1 sign or a 2 sign. The 1 sign indicates that the folder project. The function of the folders under Main Task is
is closed. Click it to expand the tree display and display to determine and specify the order in which the programs

Periodic task scan
5 ms
10 ms
15 ms Program 1
...
Program 2

Continuous task scan Periodic Program 32
task
Program 1
Task auto-restart Program 32 Continuous
Program 2
task

Figure 15-7 Continuous and periodic tasks.

336 Part 1 Memory and Project Organization

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