As PLCs have developed and expanded, programming languages have developed with them. Programming languages allow the user to enter a control program into a PLC using an established syntax. Today’s advanced languages have new, more versatile instructions, which initiate control program actions. These new instructions provide more computing power for single operations performed by the instruction itself. In addition to new programming instructions, the development of powerful I/O modules has also changed existing instructions. These changes include the ability to send data to and obtain data from modules by addressing the modules’ locations. For example, PLCs can now read and write data to and from analog modules. All of these advances, in conjunction with projected industry needs, have created a demand for more powerful instructions that allow easier, more compact,
function-oriented PLC programs.
The three types of programming languages used in PLCs are:-
The ladder and Boolean languages essentially implement operations in the same way, but they differ in the way their instructions are represented and how they are entered into the PLC. The Grafcet language implements control instructions in a different manner, based on steps and actions in a graphic oriented program.
For ease of programming the programmable controller was developed using existing relay ladder symbols and expressions to represent the program logic, needed to control the machine or process. The resulting programming language, which used these original basic relay ladder symbols, was given the name ladder language. Figure below illustrates a relay ladder logic circuit and the PLC ladder language representation of the same circuit.
The evolution of the original ladder language has turned ladder programming into a more powerful instruction set. New functions have been added to the basic relay, timing, and counting operations. The term function is used to describe instructions that, as the name implies, perform a function on data i.e. handle and transfer data within the programmable controller. New additions to the basic ladder logic also include function blocks, which use a set of
instructions to operate on a block of data. The use of function blocks increases the power of the basic ladder language, forming what is known as enhanced ladder language. The format representation of an enhanced ladder function depends on the programmable controller manufacturer; however, regardless of their format, all similar enhanced and basic ladder functions operate the same way.
Some PLC manufacturers use Boolean language, also called Boolean mnemonics, to program a controller. The Boolean language uses Boolean algebra syntax to enter and explain the control logic. That is, it uses the AND, OR, and NOT logic functions to implement the control circuits in the control program. Figure below shows a basic Boolean program.
The Boolean language is just the another way of entering the control program in the PLC, rather than an actual instruction-oriented language. When displayed on the programming monitor, the Boolean language is usually viewed as a ladder circuit instead of as the Boolean commands that define the instruction.
Grafcet (Graphe Fonctionnel de Commande Étape Transition) is a symbolic, graphic language, which originated in France, that represents the control program as steps or stages in the machine or process. In fact, the English translation of Grafcet means “step transition function charts.” Grafcet is the foundation for the IEC 1131 standard’s sequential function charts (SFCs), which allow several PLC languages to be used in one control program.
Figure below illustrates a simple circuit represented in Grafcet. Note that Grafcet charts provide a flowchart-like representation of the events that take place in each stage of the control program. These charts use three components— steps, transitions, and actions—to represent events.
The IEC 1131 standard’s SFCs also use these components; however, the instructions inside the actions can be programmed using one or more possible languages, including ladder diagrams Once programmed in the PC, the Grafcet instructions can be transferred to a PLC via a translator or driver that translates the Grafcet program into a ladder diagram or Boolean language program. Using this method, a Grafcet software manufacturer can provide different PLCs that use the same “language.”