The data block (DB) is defined in the memory of the S7-CPU, and the user can create one or more data blocks in the memory. Each data block can be large or small, but the CPU has a limit on the number of data blocks and the total amount of data. For the CPU 314, the memory used as the data block is up to 8 KB, and the total amount of user-defined data cannot exceed this limit. The data block must follow the principle of being defined first, otherwise it will cause a system error.
A data block (DB) can be used to store variable data (such as values) of logic blocks in the user program. When the logic block execution ends or the data block is closed, the data in the data block remains unchanged.
User programs can access blocks of data using bit, byte, word, or double word operations, or they can be accessed using symbols or absolute addresses.
There are three types of data blocks: shared data blocks, instance data blocks, and user-defined data blocks.
Shared data blocks are also called global data blocks. Used to store global data, all logic blocks (OB, FC, FB) can access the information stored in the shared data block.
The instance data block is used as a "private memory area" as a "memory" for function blocks (FBs). The parameters of the FB and the static variables are arranged in its instance data block. The background data block is not edited by the user but generated by the editor.
Background data blocks and shared data blocks have different uses. Any FB, FC, or OB can read and write data stored in a shared data block. The background data block is the working storage area of ​​the FB runtime. It stores part of the running variables of the FB. When calling the FB, you must specify a related instance data block. As a rule, only the FB can access the data stored in the instance data block. If there are not enough internal memory bits in the CPU to hold all the data, some of the specified data can be stored in a shared data block. The data stored in the shared data block can be used by any other block. An instance data block is assigned to a specific function block, and its data is valid only in this function block. In contrast to the instance data block, the data type of the data block in the symbol table is always an absolute address. For instance data blocks, the corresponding function block is always the specified data type.
The CPU has two data block registers: DB (to store shared data) and DI (to store background data) registers. This way, you can open two data blocks at the same time.
The water glass shell process has low cost, simple operation, and low requirements for raw materials. The basic feature of the process is the use of paraffin-stearic acid low-temperature mold material, and the use of water glass as the binder in the process of shell making. In terms of shell process, its surface quality is average, and its dimensional accuracy is not as high as that of silica sol process, but because of its unique cost, it is widely used in some civil and low-precision internal combustion engines in my country.
Sand Casting,Iron Sand Casting,Oem Sand Casting,Cast Iron Sand Casting Parts
Tianhui Machine Co.,Ltd , https://www.thcastings.com