Suggestions for newcomers to learn Siemens PLC statement table - Database & Sql Blog Articles

Single chip microcomputer STM32L151CCU6

Photocoupler

I have come across a lot of peers. They all say that the S7-300/400 statement list (the instruction list) language is difficult to learn. Recently, on the Siemens S7-300/400 forum, I saw that some netizens lamented that "the sentence list is so difficult to learn." !". Why is the ladder diagram eager to learn? The reason is very simple. The ladder diagram is derived from the relay circuit diagram, which is mainly based on contacts and coils. Therefore, the electrical personnel familiar with the relay control feel very easy to learn.
1. Statement table is not a school
1) The statement table has more instructions than the ladder diagram. Only the statement table has the following instructions: FR instruction of timer/counter, a large number of instructions related to address register, accumulator, status word, data block, loop instruction (LOOP) and Partial jump instructions, block end instructions, etc.
2) The function of the statement list is stronger than the ladder diagram. For example, the cyclic program and the indirect addressing can only be programmed with the statement table.
3) The actual control program (especially the program compiled by the foreigner) has almost all the logic blocks written by the statement table. However, statement list programs cannot be completely converted to ladder diagrams with STEP7. Blocks that cannot be converted are still represented by a statement list.
Therefore, in the actual work, the statement table is difficult to avoid. To learn S7-300/400, you must learn the sentence list.
2. Why is the statement table difficult to learn?
1) The statement table language is somewhat similar to the assembly language of Intel's 51 series MCUs. For example, they all have accumulators, status words and address registers. Ladder diagrams do not use accumulators and address registers.
2) Statement List Program Almost all operations are done in the accumulator, so a large number of load (L) instructions are needed to load the data into the accumulator, and the transfer (T) instruction is used to accumulate the result from the accumulator. Transfer to the destination address. Comparing the S7-300 and S7-200 cycle programs, you will find that the former is much more complicated.
The ladder diagram only needs one MOVE instruction, and the data can be arbitrarily transmitted without the accumulator.
2) Indirect addressing of S7-300/400 is much more complicated than indirect addressing of other plc. Only the statement table has indirect addressing function, which is one of the reasons why the statement table is difficult to learn.
3. Suggestions on learning the statement list
1) First of all, lay the foundation and master the necessary computer basic knowledge, such as binary number, hexadecimal number, BCD code, complement code, various data types, parameter types, accumulators, status words, address registers, and Basic concepts and applications such as addressing modes.
If you have learned assembly language at school, it will be good for learning the statement language. Some people suggest that to learn the assembly language in order to learn the sentence list, I think it is not necessary.
2) Multi-use and practice, in practice middle school. The language of PLC and computer is just like foreign language. It must be used more and more, mainly by reading programs, programming and debugging programs. When you are a beginner, you can find a good textbook and read the sentence list program with a lot of comments in the textbook. On this basis, you can read the utilities in production. It is best to program some of your own and then use the simulation software to debug the program.
3) Make full use of STEP7's online help function, use the mouse to select an instruction in the user program, press <F1> to open the online help of the instruction, and see the detailed use of the instruction, such as the function description of the instruction, the instruction The data type of the input/output parameters, the meaning of the memory areas and parameters that are allowed to be used, the effect of the execution of the instructions on the status word, and instances of the instruction application.