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| Hardware of Siemens |
I. Basic Concepts of Pointers
A pointer is a variable that holds the address of another variable, enabling indirect access and manipulation of data stored in memory. In PLC programming, pointers find widespread applications, such as array access, structure navigation, and dynamic memory allocation.
II. Representation of Pointers
The representation of pointers in Siemens PLCs is often tied to the specific programming environment and data types involved. Generally, obtaining a variable's address and assigning it to a pointer variable involves utilizing specific instructions or operators. For instance, in certain PLC programming environments, the "&" symbol can be employed to fetch a variable's address and assign it to a pointer variable.
III. Accessing Pointers within DBs
In Siemens PLCs, pointers within DBs are utilized to represent and access data within the DB. To access a specific piece of data within a DB, one must first determine its address. In Siemens PLCs, the address representation typically comprises elements such as the memory area identifier, the memory area size specifier, and the exact numeric unit or offset.
By leveraging pointers, efficient data manipulation and dynamic referencing within DBs are achievable, enhancing the flexibility and power of Siemens PLC programming.
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| DB programs |
- Address Composition
A typical address representation might look like the following:
DBX200.0
Here, "DB" is the memory area identifier, indicating that it is a Data Block; "X" is the memory area size specifier, signifying byte or bit addressing; "200" is the exact numeric unit, representing the offset within the data block; and "0" is the bit value, indicating the 0th bit within that byte.
- Indirect Addressing
In addition to directly specifying an address for access, Siemens PLCs support indirect addressing. Indirect addressing allows specifying the address to be accessed through a pointer variable. This enables dynamically changing the operand of an instruction during runtime, thereby accessing different data locations.
In Siemens PLCs, indirect addressing typically falls into two categories: memory-based indirect addressing and register-based indirect addressing.
Memory-based Indirect Addressing: Specifies the address to be accessed through a pointer value stored in the M, DB, DI, or L areas. The pointer can be a single-word pointer (16-bit) or a double-word pointer (32-bit), each suited for different addressing requirements.
Register-based Indirect Addressing: Utilizes dedicated address registers (such as AR1 and AR2) to store pointer values. By combining address registers with pointers, access to more complex address structures can be achieved.
IV. Example of Pointer Application
Below is a simplified example of using a pointer to access data within a DB:
PROGRAM MAIN VAR ptr: POINTER TO INT; // Defines a pointer variable pointing to an integer // Assumes DB1.DBX0 stores an integer value valueInDB: INT AT DB1.DBX0; // Used to store the value read from the DB readValue: INT; END_VAR // Assign the address of DB1.DBX0 to the pointer variable ptr ptr := ADR(valueInDB); // Note: ADR and ^ operators are hypothetical here for illustration // Indirectly access the data in the DB through the pointer ptr and assign it to readValue readValue := ^ptr; // Hypothetical usage of ^ for dereferencing // At this point, readValue will contain the integer value stored in DB1.DBX0
Please note that the ADR and ^ operators used in the aforementioned example are hypothetical and serve as illustrations to demonstrate how data within a DB might be accessed through a pointer. In actual programming, you will need to select the appropriate instructions or operators based on the PLC programming environment and language you are utilizing.
V. Precautions
When utilizing pointers, it is imperative to ensure that the address pointed to is valid and that any operations performed on that address are safe.
Pointer operations can potentially increase the complexity of your program, thus they should be employed judiciously during program design.
Furthermore, when performing pointer operations, attention must be paid to data type and address alignment issues to guarantee proper access and modification of data.
In summary, pointer access to DB data structures in Siemens PLCs represents a potent programming technique that enables programmers to access and manipulate data in memory with greater flexibility. By leveraging pointers judiciously, program efficiency and maintainability can be enhanced.