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HOME >> Introduction of Siemens S7-300 / 400 PLC ob-fb-fc-db block
Introduction of Siemens S7-300 / 400 PLC ob-fb-fc-db block
Source: | Author:pmo89cafb | Published time: 2020-10-26 | 1790 Views | Share:

In the block FC, there are some logic blocks which are provided by PLC manufacturers and integrated in the S7 CPU operating system, called system function (SFC). System block SFC is a part of PLC internal operating system. Users do not need to write or edit it, but they can be called directly according to their needs.

Function block (FB)
Function blocks (FB blocks) are commonly used logic blocks written by users and supported by instance data blocks (DI).
The function of FB block and FC block is basically the same, but in FB, in addition to using "absolute address" or "symbol address" for programming, the "program variable" must be used for programming in structured programming. Therefore, FB must be a supporting and independent data storage area - "instant data block Di". On the other hand, the function block FB can also return the required data to the logical block calling it through di.
Like FC, there are some function blocks in FB which are provided by PLC manufacturers and integrated in S7 CPU operating system, which are called system function blocks (SFB). System function block SFB is also a part of PLC internal operating system. Users don't need to write or edit it, but they can be called directly according to their needs.