1756-PM003H-EN-E, SFC and ST Programming Languages ...

Design a Sequential Function Chart 4-5
Define the Tasks
The first step in the development of an SFC is to separate the
configuration and regulation of devices from the commands to those
devices. Logix5000 controllers let you divide your project into one
continuous task and multiple periodic tasks and event tasks.
1. Organize your project:
These functions:
configure and regulate devices
command a device to a specific state
sequence the execution of your process
Go here:
periodic task
SFC in the continuous task
2. For those functions that go in a periodic task, group the
functions according to similar update rates. Create a periodic
task for each update rate.
For example, 2-state devices may require faster updates than
PID loops. Use separate periodic tasks for each.
In this example, a project uses two periodic tasks to regulate motors,
valves, and temperature loops. An SFC to control the process.
EXAMPLE
Define the Tasks
This task (periodic) uses function block diagrams to turn on or
off motors and open or close valves. The SFC in MainTask
commands the state for each device. The function block
diagrams set and maintain that state.
This task (periodic) uses function block diagrams to configure
and regulate temperature loops. The SFC in MainTask
commands the temperatures. The function block diagrams set
and maintain those temperatures.
This task (continuous) executes the sequential function
chart (SFC). The SFC commands the specific state or
temperature for each device or temperature loop.
Publication 1756-PM003H-EN-E (excerpt from 1756-PM001H-EN-P) - August 2005