IEC-defined programming languages

IEC 61131-3 describes three graphical and two textual programming languages and defines their language elements as well as their syntax.

Ladder Diagram (LD)

The Ladder Diagram language (LD) has been used since the invention of PLCs for their programming. LD is suited to programming sequential logic and for this reason is accepted worldwide and can be used universally. Development engineers as well as service engineers are capable to analyze the LD programming of a controller on site and diagnose and troubleshoot problems if required.
IEC 61131-3 defines a complete LD set of instructions, consisting of different types of contacts and coils. Contacts lead (according to their type) the power from the left to the right. Coils store the incoming value. Both, contacts and coils, are assigned to Boolean variables. An LD network can be supplemented by jumps, returns, labels and comments.

In addition, MULTIPROG^® provides the opportunity to use FBD elements in LD networks, as shown in the following example.

Function Block Diagram (FBD)

The language Function Block Diagram (FBD) defined in IEC 61131-3 consists of different functions and function blocks that are either linked or connected to variables. In FBD networks, jumps, labels and returns can be used. Apart from standalone use of FBD, LD elements can be connected to timers, counters and type converters etc. Programming is similar to the creation of functional diagrams or block diagrams, with the control logic being created by simply connecting blocks and elements.

FBD is frequently used in the process industry due to the flow-oriented properties of function blocks (reading FB inputs, processing, setting FB outputs).

Instruction List (IL)

Structured Text (ST)

Sequential Function Chart (SFC)

SFC is a status-oriented language. The code body programmed in the graphic language SFC consists of steps and transitions. A step contains the logic for a certain machine status, a transition represents a condition under which processing of the code is continued with the next step.
SFC is particularly suited to applications that can be structured into clearly identifiable steps. The logic contained in a step can be programmed in any other IEC programming language, adjusted to the conditions of the respective machine status. This way, SFC becomes the first choice particularly for commissioning and the debug phase, as the structuring of the application into single steps significantly simplifies program diagnosis in case of errors - especially compared to a typical LD program with a large number of networks.