Rockwell Automation 57C570 AutoMax PC3000 User Manual - Manual (Page 172)

15Ć2
A A
Task B is
Interrupted
by Task A
Task A Ć Every 5 milliseconds, 2Ćmillisecond duration, priority = 5
Task B Ć Every 10 milliseconds, 4Ćmillisecond duration, priority = 6
NOTE: In AutoMax DCS, lower numeric value = higherĆpriority assignment,
i.e., 5 is higher priority than 6.
0 5 10
Figure 15.1 Ć AutoMax MultiĆTasking
Any process that is to be controlled can be broken down into small
application programs. Each program may control an individual
function in software, but is linked to other programs to control the
overall process. For example, in a batching operation, control is
needed to measure and weigh the different elements of a specific
mixture. This can be handled in one software module or task. In
addition, a need for some type of machineĆsequencing control
usually exists. This would make up a second software block.
Operator interface and data collection are two more functions that
could be a part of the system. As the main system is broken down
into its smaller components, approaching the control of each of
these individual components becomes easier than trying to solve
the entire application in one effort.
These programs, although separate, share control information
across their boundaries. They reference the appropriate variables by
name, and the CPU can access the I/O and memory associated with
the names.
MultiĆtasking helps you simplify the writing, debugging, and
maintaining of your application program by letting you separate your
overall control scheme into individual programs. MultiĆtasking also
reduces the overall execution time of the control scheme and
provides a faster response to critical programs. You can also stop a
program while others are running, facilitating troubleshooting and
testing.
15.1.2 About UserĆScheduled Programs
After dividing your application into logical parts and writing
programs to control each part, you can schedule these programs to
run according to the needs of the application. With priority
scheduling in a multiĆtasking environment, you can make sure that
more critical programs are not suspended by the execution of lower
priority programs.
You can assign a program a priority level of 4Ć11, where 4 is the
highest priority level and 11 the lowest. As different programs are
executed, higher priority programs can interrupt" lower priority