This article is about the FB block which we use in Siemens PLC programming.
Function block (FB) is used to make the programming task easier. It is like subroutine which we have seen in Allen-Bradley PLC.
To stop writing the program over and over, it is a good practice to write the program in Function Block and call that program on-demand from OB.
The function block is configured to accept input from the main OB and write the output based on the execution of the FB.
FB is like FC with one difference in that is FC has overall memory space and doesn’t require any extra block to store that value whereas in FB it requires DB (Data Block) to store values. For each new FB, a new DB is required.
To understand how FB works, I will explain it using a simple example.
Let’s first learn how to create FB, to do that so follow the below steps.
Let, write PLC program for pet bottling process, where a bottle is getting rejected due to some issue like the cap is torn, weight is less or the cap is not properly fitted, etc.
The rejected bottle is automatically sent to the rejection conveyor. Now rejection ratio is too higher which fills till a proximity sensor activates in terms of sound alarm.
Let, assume we have 4 to 5 same rejection conveyor and I want to write the program. Now instead of writing the same logic over and over I will write the program in FB and call each time when OB executes.
A reset button is used to reset the alarm. Here, I have used alarm_1 as a latching in both input and output side because you can’t define output ALARM as input in FB block.
As soon as you move FB to OB you will receive a pop-up as shown in the below window.
Here, you have to define the DB block as a memory space of FB. Click on “ok” to add DB.
We can add other conveyor’s input and output here and each time any input gets activated, it will call FB and alarm will sound. Resetting the output will reset the alarm.
Author: Suhel Patel
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