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IEC 1131


   What Is IEC 1131?
   Ladder Diagram
   Sequential Function Chart
   Function Block Diagram
   Structured Text
   Instruction Lists
 

IEC 1131 Sequential Function Charts
SFC programming offers a graphical method of organizing the program. The three main components of an SFC are steps, actions and transitions. Steps are merely chunks of logic, i.e., a unit of programming logic that accomplishes a particular control task. Actions are the individual aspects of that task. Transitions are the mechanisms used to move from one task to another. Control logic for each Step, Action and Transition is programmed in one of the other languages such as Ladder Diagram or Structured Text.

As a graphical language, SFC programming offers you several choices for executing a program, each depicted in a visually distinct way (Fig. 1). In a sequential configuration, the processor simply executes the actions in step 1 repeatedly, until the transition logic becomes true. The processor then proceeds to step 2. In a selection branch, only one branch is executed depending on which transition is active. In a simultaneous branch, all branches are executed until the transition becomes active. In addition to various types of branches, the operation of individual actions within a step can be varied with the use of action qualifiers.

SFC Selection Branch
FIGURE 1

Action qualifiers (Fig. 2) determine how the action is scanned and allow actions to be controlled without additional logic. For example, one could use the L qualifier to limit the time that ingredient valve B is opened.

SFC Simultaneous Branch
FIGURE 2

In practice, an active step is highlighted to signal to the programmer which part of the program is executing - a useful feature for troubleshooting. This highlighting is an example of the standard's extensibility - the ability of a vendor to add a feature not specified in the standard.

Note that the standard offers SFC programming as an organizing tool. The user chooses whether to use it or not, based on whether the process being controlled is sequential in nature. And even if SFC programming is used, the actions will be written in one of the four programming languages described below. Figure 3 shows a sample net weight calculation as it would be performed in each of these languages. In each example, net weight is calculated by subtracting tare weight from the gross weight.

SFC Sequential configuration
FIGURE 3

SFC Action Qualifiers

SFC Qualifyer
Description
N Nonstored. Terminate when the step becomes inactive.
S Set (stored). Continue after the step is deactivated, until the action is reset.
R Reset. Terminate the execution of an action previously started with the S, SD, SL, or DS qualifier.
L Time Limited. Start when step becomes active and continue until the step goes inactive or a set time passes.
D Time Delayed. Start a delay timer when the step becomes active. If the step is still active after the time delay, the action starts and continues until deactivated.
P Pulse. Start when the step becomes Active/Deactive and execute the action only once.
SD Stored and time Delayed Action starts after time delay, continues until reset.
DS Delayed & Stored. If step is still active, action starts after time delay, continues until reset.
SL Stored & timeLimited. Action starts when step becomes active, continues for a set time or until reset.