AdvanTech Corporation

IEC-1131-3 is the international standard that specifies the syntax and semantics for programming an indigent controller (PLC or DCS). The standard incorporates 5 different languages. One of the benefits of this standard is that it allows multiple languages to be used within the same programmable controller. This allows the program developer to select the language best suited to each particular application or task.
IEC-1131-3 defines the following 5 programming languages:

Ladder diagram (LD)

LD is the best-known PLC programming language developed for users who understand traditional relay-based controls. LD continues to be one of the most popular PLC programming languages in the United States. Although it is possible to program all control logic in LD, supplementing LD with other languages provides the convenience of using languages that are best suited for a particular control task.


Sequential Function Charts (SFC)

SFC programming offers a graphical method of organizing the program. The three main components of an SFC are steps, actions and transitions. A step is a grouping of logic used to carry out a particular control task. Actions are the aspects of that task. Transitions are mechanisms used to move from task to task. Control logic for each Step, Action and Transition is programmed in any of the other languages.


Function Block Diagram (FBD)

Like SFC, FBD is a graphical language that allows programming in other languages (ladder, instruction list, or structured text) to be nested within the FBD. In FBD, program elements appear as blocks, which are "wired" together in a manner resembling a circuit diagram. FBD is most useful in those applications involving a high degree of information/data flow between control components, such as process control.


Structured Text (ST)

ST is a high-level language that is reminiscent of Pascal or Basic. ST is ideal for tasks requiring complex math, algorithms or formulas. Often ST program lines resemble sentences, making it comprehensible to the novice users.


Instruction List (IL)


IL is a low-level language similar to Assembly language. While very powerful, IL is not used very often due to its complexity.