Function Block Diagram Programming


Thread Starter

Clint Sanders

Could anyone tell me which plc software packages include function block diagram programming and the company that is manufacturing the software and plc. The problem is that at my company there are 3 programmers, 2 of which favor ladder logic and 1 who favors fbf. Ideally, I am looking for a plc that can be programmed in ladder logic or fbd. Would also like to hear opinions from
everyone about which type of programming tool they favor & why. If anyone knows of a good resource for learning fbd, that would be appreciated as well.
Rockwell Software's RSLogix 5000 supports both, it programs Allen Bradley's ControlLogix and other "Logix" processors.

Schneider Automation has at least one product that does both - not sure about Telemecanique, but Concept software (or whatever it is called these days) for Modicon PLC's has it.

Most of the time there are different versions of the programs, so be sure you have purchased the options that allow you to do ladder or function blocks.

As to preference, depends on the end user, in my opinion. Who will be supporting the program after it is written? What would they prefer to see? We have stuck mainly to ladder simply because that is what our customers are familiar with (although I don't think function block would strain their brains).
Siemens S5 has a feature that you can switch back and forth. It doesn't matter what format it was programmed in. That way the end user can
view with whatever they are comfortable with. I haven't had a chance to use Siemens S7 yet, but I would be suprised if they didn't carry
over this feature.

Naveen Nagar

I strongly believe that "LADDER LOGIC" is for electricians while "FUNCTION BLOCK DIAGRAM" is THE Language for Engineers. "STEMENT LIST" is
for Experts (Nerds)

Siemens packges STEP-5 ( For SIMATIC S5 ) and STEP-7 ( Form SIMATIC S7 ) support all the three formats
Concept from Schneider Automation can program all
5 of the IEC61131 PLC languages. It can be used with Quantum, Momentum, and Compact, Modicon processors. The 5 Languages are Ladder Diagram
LD, Function Block Diagram FBD, Structured Text (Like Pascal), Instruction List IL (like assembly language) and Sequential Function Chart SFC (like a state machine or "Flowchart"language). It will also program the legacy 984 ladder logic we know and love.

ControlLogix processors from AB also program in Ladder and FBD and I think SFC with the other languages coming soon.

I prefer FBD over all. It is much more readable than ladder especially with large programs.
Telemechanique (also from Schneider) uses PL7 software to program their "Premium" PLC which
is directed at the same market as ControlLogix.

I believe it has all the languages as well but not as well implemented as Concept IMHO. (It does do wicked things with motion control
though). PL7 includes all the bells and whistles including a Human Machine Interface program (HMI) that you can create control panels for the maintainer to look at while troubleshooting. The best part is that you do all of this without leaving the same application space. Schneider is working on a merge of the best of Concept and PL7.

David Lawton

As mentioned, Siemens (S7 as well as S5) and Modicon Concept, I also believe Giddings & Lewis has FBD. Not totally sure RSLogix5000 has
it(?). I'm sure FBD has it's uses, but in my 10 years+ experience I've always programmed in ladder. It's what customers ask for and
electricians generally find it easier to understand when fault finding.

Jeremy Pollard

Hi Clint. IEC61131 supports both. Go to for a vendor list. Rockwell has a FB product - RSFrameworks. Also ladder of course. Can the two co-exist? Not sure of the hardware platform other than the CL 5000. Modicon with Quantum for sure.

Open control solutions abound as well. Remeber that Function block has to have code underneath it as in SFC. So the FB guy could lay out the architecture, and the coders can write the RLL.

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Siemens Step 7 for the S7 range of PLC's includes both Ladder and Function Block Programming. (Statement List as well).
Personally, I prefer to program in ladder but have recently come across an application that would be better suited to progamming in a
Function Block format. Siemens have a Function Block progamming manual available for download from their website.


Modicon's Concept support LD and FBD.

My opinions here refer to Concept only, I have not used other FBD implentations.

FBD is much easier to write and much easier for someone else to read. I have never even had the slightest temptation, when programming in Concept, to write a section in LD.

FBD essentially cuts out the electrical middleman and lets you express your solution to a control problem in a more natural format - something like a boolean logic diagram which is often used to specify controls.

Another huge advantage to FBD is it is much more compact than ladder. On a single screen I can see the entire solution to most problems. No more paging back and forth or up and down to see enough LD to get a picture of what's going on. Also FBD reduces the need to intermediate coils and registers that are often required by the limitted work area provided by LD.

To me going from ladder to FBD was like going from a line editor to a WYSIWYG editor.

Jay Kirsch

The SCADAPack PLC by Control Microsystems ( ""; ) the
ControlWave PLC from Bristol Babcock ( "": ) offer all five IEC61131-3 Programming languages. In fact the BBI unit can be programmed
in one or all five of the languages. This allows you to use the best programming language for the best application. Who says you can't have
your pie and eat it too!!!


Bob Dusza::

Robert J. Dusza, Jr.
Project & Technical Support Manager
(V) 1-860-647-3219
(F) 1-860-647-3150
E-mail - [email protected]
Manchester Water & Sewer Dept.
125 Spring St. P.O. Box 191
Manchester, CT 06045-0191

Michael Griffin

Programming software for Siemens S7 PLCs (S7-200 and S7-300/400) supports both ladder and FBD. The programming software will toggle between the two display modes. This is, you could select either FBD or ladder and the same logic will be displayed in either format.
However, it is possible to write ladder rungs that will not translate into FBD. It will be quite common in fact. I do not know if the reverse is true (FBD not translating to ladder). This is strictly a display problem, and you
won't lose part of your program. You can always simply switch to the mode the program was written in to see it in the original format.

I suspect that this problem is inherent in FBD and ladder formats. If this is acceptable, then you may have a solution.


Michael Griffin
London, Ont. Canada
Hi Michael,

That's very interesting. Modicon's Concept is not designed to toggle between FBD and LD as if they were different representaions of the same problem. In Concept, the FBD is a higher level language than LD. FBD could, in theory, be re-engineered into an LD format, but not the other way around. Even if Modicon did this the result would be gibborish, like transforming Fortran-IV into Ada. ( Don't think about this, reverse engineering is now a thought crime ).

What's going on here ? The original poster wants to know which is better, FBD or LD, yet there seems to be a huge gap in the way FBD is implemented among these two vendors. Has anyone used both the Modicon's and Siemen's FDB implementations ? Maybe we could exchange
some pdf's with specific examples of these FBD implemations. For now, it makes no sense to me whatsoever, to toggle between FBD, as I know it, and

Jay Kirsch
ABB's AC800M, AC800C controllers and the corresponding Control Builder
software program in all five IEC 6-1131 languages: Ladder Diagram,
Structured Text, Function Blocks, Instruction List, and Sequential Function
Charts. You can even mix all the languages within the same controller if
you want.

Johan Bengtsson

As for training our computer based course in PLC programming might be what you are looking for.
Check our homepage at "":

/Johan Bengtsson::

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E-mail: [email protected]
I am reviewing various systems trying to determine our future path for SCADA. We want to find an alternative to the Siemens TI555 PLC programmed with APT. Have you used the ABB Control Builder, if so, what are you using for graphical interface (another ABB software, or?) Can you give me a clue about what kind of process you are controlling and what the control architecture looks like, and how satisfied you are with Control Builder, as well as any observations about its strengths and weaknesses.