Common variables and local variables of Siemens PLC - Database & Sql Blog Articles

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1 variable V (S7-200 only): In S7-200, the internal variable of plc is marked with the English letter V. Its function is similar to the internal register. It can be used to register the intermediate operation result of the PLC program, but it can be used more. The contents of the internal variable V can be continuously updated and changed during the execution of the PLC program. Since the state of the variable V can be shared by all the blocks, it is a public variable (also called a shared variable).
Since there is no separate "data block (DB)" memory area in the S7-200, the first part of the internal variable V memory is used as the data block DB1, which varies in size from different CPUs and can be 128 to 512 bytes. . The internal variable V used as the data block DB1 can be copied to the PLC EEPROM like the PLC program when the PLC is loaded, and the remaining internal variables V can only be saved in the CPU RAM.
In the S7-300/400, the internal variable V cannot be used, but the intermediate operation result of the PLC needs to be stored by the data block DB.
2 Local variables L: PLC internal local variables are marked with the English letter L, which acts like a public variable, but it is a shared, dynamically changing storage area whose content and meaning vary with the logic block being executed. Change, which is used to register intermediate operations that are only used inside the logic block.
The difference between the local variable L and the public variable V is that the common variable V is common in the PLC program, that is, the execution result of a certain program segment or program block can be used for other program segments and program blocks; and the content of the local variable L It is temporary and not fixed. Only when a certain logic block is called, there is a clear meaning inside the block. Once the logic block is executed, its role disappears. Therefore, it follows the executed block. Update and change at any time.
The address range of the local variable L is theoretically 64 bytes from LO.O to L63.7, but the actual usable range is generally 60 bytes from LO.O to L59.7.