Rockwell Automation 57C610 Enhanced Basic Language, AutoMax - Manual (Page 14)

2Ć2
The configuration task does not actually execute or run; it serves as
a central storage location for systemĆwide information. Note that
local variables, those variables that do not need to be accessible to
more than one task, do not need to be defined in the configuration
task. Refer to JĆ3649 for more information about configuration tasks.
2.2 AutoMax Application Tasks
AutoMax Processors allow realĆtime concurrent operation of multiple
programs, or application tasks, on the same Processor module. The
tasks are executed on a priority basis and share all defined system
data. Application tasks on different Processor modules in the rack
are run asynchronously.
Each task operates on its own variables. The same variable names
may be used in different tasks, but each variable is only recognized
within the confines of its task unless it is specifically designated a
COMMON variable. Changing local variable ABC% (designated
LOCAL) in one task has no effect on variable ABC% in any other
task.
MultiĆtasking in a control application can be compared to driving a
car. The programmer can think of the different functions required as
separate tasks, each with its own priority.
In driving a car, the operator must monitor the speedometer,
constantly adjust the pressure of his foot on the gas pedal, check
the rearview mirror for other traffic, stay within the boundaries of his
lane, etc., all while maintaining a true course to his destination. All of
these functions have an importance or priority attached to them,
with keeping the car on the road being the highest priority. Some
tasks, like monitoring the gasoline gauge, require attention at
infrequent intervals. Other tasks require constant monitoring and
immediate action, such as avoiding obstacles on the road.
In a control application the Processor needs to be able to perform
calculations necessary for executing a control scan loop, monitor an
operator's console, log error messages to the console screen, etc.
Of these tasks, executing the main control loop is obviously the
most important, while logging error messages is the least important.
MultiĆtasking allows the control application to be broken down into
such tasks, with their execution being dependent upon specified
events," such as an interrupt, operator input, or the expiration of a
time interval.
The following table is a representation of typical tasks found in a
control application and the kind of event that might trigger each.
Task Triggering Event
Execute main control loop Expiration of a hardware
timer that indicates the
interval at which to
begin a new scan
Respond to external I/O input Generation of a hardware
interrupt by an input module
Read operator data Input to an operator panel
Log information Expiration of a software timer