from the Automation List department...
Understanding Function Blocks vs Ladder Logic
PLCs and related questions. topic
Posted by Mike Staveskie on 3 April, 2009 - 1:18 pm
I was given a program to execute explicit messaging between a safety plc and a plc. The program was all in function blocks which I have never learned before, but I understand ladder logic. My issue is that I am trying to monitor additional switches for my application and this program includes only three dual channel inputs. Any help would be greatly appreciated

Posted by Curt Wuollet on 4 April, 2009 - 4:01 pm
Function blocks are really easy if you understand digital logic circuits. They are strongly similar to designing with ICs. AND gates, OR gates, flip flops, and special functions. Any digital logic or electronics text will go a long way to help. They can be a little more difficult as the logic is more dense than ladder.

But by simply understanding how each function works you can get into it. The way I would start with either is identical. Simply write out the logic in human terms then reduce to pseudocode. If the gas is on and the pilot is out and the temperature is high then shutdown. The if ....then is implicit so the function would then become 3 NO contacts and a coil. In function block, a 3 input AND with a coil.


Posted by Mike Staveskie on 9 April, 2009 - 8:25 am
My issue is with cx programmer. I'm not sure how to set up the function blocks like I was given. I am trying to find a tutorial but no luck. The program that was sent is using 3 dual channel switches, I want to include a couple of single channel switches but I am not having any luck. I think its an addressing issue. Any suggestions?

Posted by padli on 5 April, 2009 - 10:49 pm
Although function block diagram has been specified through IEC 1131, PLC and DCS vendors have their own interface FBD graphics.

To see the status of switch, you can find the appropriate diagram in the library of your engineering station. If there is no specific diagram listed, you can use AND block with 2 input. One input for your signal and another connected to constant "true".

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