Siemens PLC temporary variable L - Solution - Huaqiang Electronic Network

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1 The local data stack area L is a local variable, and the L variable is valid only in the subroutine in which it is called.
The L variable in different subroutines can have the same name. For example, there is L0.1 in FC1 and L0.1 in FC2, and there is no relationship between the two, and they will not conflict with each other.
2 does not need to be initialized
3 local variable interpretation:
L is a local memory. As a temporary memory or a parameter for a subroutine, L can also be L*.* (if L2.5 is the BOOL quantity, which is the fifth bit of the second byte), LB, LW, LD Take, but only valid in the block it is created, it can also be defined in the symbol table, but can only be defined in each POU's own symbol table.
4. Global Variables and Local Variables In symbolic addressing programming, global variables directly display symbolic names, while local variables are preceded by # in the symbol. There are many advantages to using local variable programming: programs can be functionalized (reduced reprogramming) ), large programs facilitate collaborative operations (dividing tasks into functional blocks), which is the superiority of SIEMENS's plc.
5 Please note that you must first assign a value, then use a local variable, also known as a temporary local data area (L stack), located in the working memory area of ​​the CPU to store temporary data when the program block (OB, FB, FC) is called. Accessing temporary data is faster than accessing data in a data block.
"L is a local variable, can only be used locally, can not be used globally." It can only be used in this block, it will be automatically reset after use, it can not be used by other programs. The principle of using temporary variables is to assign values ​​first, and then use them in the S7-300 CPU. The size of each priority local data area is fixed. Generally, the program block (FB, FC, etc.) is called in the organization block. The maximum number of local data areas allocated by the operating system to each execution level (organization block OB, generally executing and calling other FBs and FCs in the OB block) is 256B ( Byte), the organization block OB itself takes 20B or 22B, and the remaining 234B can be allocated to the FC or FB. If the number of local data defined in the block is greater than 256 bytes, the block will not be downloaded to the CPU. During the download process, an error message is displayed: The?block?could?not?be?copied (the block cannot be copied). If you click the Detail button in the error message box, the help message will pop up: Incorrect?local?data?length (incorrect local data length).
Expand your project in turn to Blocks in the SIMATIC Manager, right-click on a block and select the menu Object?Properties option. In the properties dialog of the block, you can see the number of local data bytes required by the block (Length length: Local? Data).
Select the Block of your project in the SIMATIC Manager (the folder (execute the menu command Options/Reference?Data/Display) and select Program?Structure. View the occupancy of local data in the reference table.