Introduction to the PLC programming language

Programmable controller at present commonly used programming language has the following kinds: ladder diagram language, mnemonics, sequence function diagram, functional block diagram and some high-level language. Handheld programmer to use mnemonics more language, computer software programming using ladder diagram language, there are also using sequence function diagram, function block diagram. ( 1) Ladder diagram language expression of ladder diagram follows the original electric control system of the relay contact control circuit diagram of the form, the basic idea of the two is consistent, just use symbols and expressions. 【 Case 1 - 1 】 A process control system, the technological requirements switch 1 closed after 40 s, indicator, press the switch 2 rear light go out. Control is realized by using mitsubishi FX2N series PLC controller, FIG. 1 5 ( a) To achieve this function of ladder diagram program, it is composed of several steps, each output elements constitute a cascade, and each step can be composed of multiple branch. Ladder diagram from top to bottom line according to the writing, each row is written from left to right order. The CPU will press from left to right, from top and bottom of sequential programs. That ladder diagram to the left of the vertical line, bus ( Source bus) 。 Ladder diagram on the left side of the arrangement of the input contact ( If there are several contact phase parallel branch should be arranged in the left side) And the auxiliary relay contact ( The intermediate results. And most the right side must be output element. The ladder diagram of the input contact only two categories: dynamic contact and moving contact ( ) , these contacts can be a PLC external switch corresponding internal image contact, can also be a PLC relay contact inside, or contact of the internal timer, counter. Every contact have their own special number, to show the difference. The same number of contact can have often opened moved off two states, using the number of times unlimited. For ladder diagram of 'relay' within the corresponding PLC used in a byte, or a storage area, the contact state, corresponding to the bit can be read again and again, so people have infinite to contact said PLC. Ladder diagram can contact any of the series and parallel. Ladder diagram of the output corresponding to the corresponding bit of the PLC memory coil output coil includes output relay coil, auxiliary relay coil and coil of counter, timer, its logical action only after connecting a coil, the corresponding contact might happen. User program operation result can be immediately to the follow-up program. ( 2) Mnemonics language mnemonics is also called the command statement expression language, some commonly used its mnemonic some operation of PLC. It was similar to the microcomputer in assembly language, but is more straightforward than assembly language. The user can easily be ladder diagram language into mnemonics. Figure 1 - 5 ( b) For the ladder diagram corresponding expressed with mnemonic instruction sheet. Here to illustrate the different manufacturer production of PLC of the mnemonics used each are not identical, so the same ladder diagram the mnemonics written statement is not the same. User in the ladder diagram into mnemonics, you must first understand PLC model and internal each device number, scope of use and the use of each mnemonic method. ( 3) Sequence function diagram sequence function diagram are commonly used to sequence control program, it includes step, movement, transformation, three elements. Sequence function diagram method can be a complex control process into a few small working condition. For these again after the function of the small state processes in turn connect these small state in accordance with a certain sequence control requirements into a combination of the whole control program. Figure 1 - 6 for the sequential function chart of program segments, see project ten more. ( 4) Function block diagram function block diagram is a programming language, similar to the digital logic circuit with similar to door, the door or a box of logic relations, the square on the left side of the input variables for a logic operation, the output variable is on the right and the input, output, the dot said 'not' computing, signal flow from left to right. Similar to the circuit, the box was 'conductor' together. Figure 1 - 7 for example function block diagram.
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