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. For instance, PLCs can now transfer blocks of data from one memory location to another while, at the same time, performing a logic or arithmetic operation on another block.
As a result of these new, expanded instructions, control programs can now handle data more easily.
There three types of programming languages used in PLCs are:
• Ladder
• Boolean
• Grafcet
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 graphicoriented program.
· Ladder language:
The ladder diagram has and continues to be the traditional way of representing electrical sequences of operations. These diagrams represent the interconnection of field devices in such a way that the activation or turning ON, of one device will turn ON another device according to a predetermined sequence of events .Figure 1-8 shown illustrates a simple electrical ladder diagram.
Figure 1-8 simple electrical ladder diagram
The original ladder diagrams were established to represent hardwired logic circuits used to control machines or equipment. Due to wide industry use, they became a standard way of communicating control information from the designers to the users of equipment.
As programmable controllers were introduced, this type of circuit representation was also desirable because it was easy to use and interpret and was widely accepted in industry. Programmable controllers can implement all of the “old” ladder diagram conditions and much more. Their purpose is to perform these control operations in a more reliable manner at a lower cost. A PLC implements, in its CPU, all of the old hardwired interconnections using its software instructions.
This is accomplished using familiar ladder diagrams in a manner that is transparent to the engineer or programmer.
The above Figure to a PLC format. Note that the “real” I/O field devices are connected to input and output interfaces, while the ladder program is implemented in a manner, similar to hardwiring, inside the programmable controller
(i.e., soft wired inside the PLC’s CPU instead of hardwired in a panel).
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