Figure 1: PLC CompactLogix Allen-Bradley supplied for a municipal project for the city of Tonka Bay, MN.
Compared with the general computer programming language, programmable logic controller (PLC) programming language has obvious characteristics. It is different from both high-level languages and general assembly languages. It is both easy to write and easy to debug. Currently, there is no programming language compatible with products from all manufacturers.
IEC 61131-3 is an international standard that regulates PLC-related software and hardware standards. Its ultimate purpose is to allow PLC users to easily replace PLC hardware without changing the software design. IEC 61131-3 mainly provides five programming languages, namely ladder diagram (LD), sequential function charts (SFC), function block diagram (FBD), structured text (ST) and instruction list (IL). This article will briefly introduce these five languages.
1. Instruction List
The instruction list (or statement list, SL) is similar to the description text of assembly language. It is composed of a series of command statements, such as the control commands LD, LDI, AND, ANI, OR, ORI, ANB, ORB, MMP, MMS and OUT of Mitsubishi FX2. Generally, the program is written with the writer, and the writer can only input simple instructions. Compared with the ladder diagram in the computer program, it is simple. Many writers are not intuitive and have poor readability, especially when encountering more complex programs. But its advantage is that you can view or change the PLC internal program without a computer.
When using the writer, it must be noted that there must be a priority order in the PLC command output. Among them, if there is output to the same unit (such as Y000), the priority of the output is higher with the larger address. The input unit is generally not easily discernible from the writer.
2. Structured Text
The structured text language is similar to the syntax of Programme d'Aide aux Sciences et aux Langues (PASCAL) and C language, and is suitable for writing more complex algorithms. It is also much easier to debug than the ladder diagram. ST language is similar to programming language. Therefore, it is possible to use the same programming technology as that of microcomputers and personal computers to perform complex calculations that are difficult to perform in ladder languages, and complete the establishment of programs.
Figure 2: An example of structured text language programming.
3. Ladder Programming
The ladder diagram language is similar to the ladder diagram traditionally controlled contactors with relays. The ladder diagram is a connected diagram that connects the ladder diagram symbols of the PLC instructions together through the online to express the PLC instructions used and their sequence. It is very similar to an electrical schematic.
4. Sequential Function Chart
The sequential function chart language is similar to Flow Design. The flow chart is mainly to plan the sequence of actions, so it is called sequential function chart. The so-called step-by-step control means that one step is related to the previous step and is sequential. There must be a previous action before starting the next action.
5. Function Block Diagram
As for the function block diagram language, we write a PLC program by drawing a circuit diagram. Commonly used programs and loops can be easily reused through the creation of function blocks (FB).
Figure 3: An example of function block diagram language programming.
Some other high-end PLCs also have computer-compatible C language, BASIC language, special high-level language (such as Siemens' GRAPH5, Mitsubishi's MELSAP, Fuji Electric's Micrex-SX series), as well as Boolean logic language, general-purpose computer Compatible assembly language, etc.
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