2851617310@qq.com
   
+86 18005007289
Hongfeida Automation
 
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 | 1793 Views | Share:

Different CPU types of S7-300 / 400 can select and support different ob blocks. However, ob1 is the cycle control block of all PLC user programs, which is the prerequisite for running PLC user programs. Therefore, any PLC program and any CPU can not lack ob1.
The call condition of OB block is called "trigger event". According to the different "trigger event", OB block can be divided into several levels. Each level has different priority. The higher priority ob can interrupt the execution of low priority ob. If necessary, S7 Series PLC can use more than one ob block in addition to ob1.、


Sequence block (FC)
Function (FC) is a common logic block written by users and does not need special data block.
FC block can not be called repeatedly in the program. In most cases, FC block should be programmed directly with PLC's "absolute address" or "symbol address". However, some program variables can be defined according to needs.
Like S7-200 -, the "temporary variables" of FC block are also stored in the local variable data stack (L). This area is common to all program blocks, and can only be used to store the intermediate operation results used inside the FC block (these intermediate operation results cannot be used outside the FC block); after the program block is executed, the data in the local variable data stack will be required by other blocks replace. If you need to save the state that can be used for other logical blocks, use the PLC's internal flag register m or use "block DB".