Is it possible to seal in coils in FBD (Logix 5000 specifically)

Hi all,

For as long as I've been programming Allen Bradley PLCs, I've pretty much stuck to ladder logic but I am interested in what function block diagrams might be able to offer.

One thing I've been trying to figure out is how one would seal in coils in FBD versus LD and after some google searching, I haven't found anything that has been very helpful. What I've found does seem to show the idea with AND blocks but it seems somewhat convoluted and more "busy" visually than ladder diagrams so it seems like it's not worth the effort. Thus I'd like to ask y'all if this is possible and if so, how would you go about it and maybe I'm missing something that would make FBD sealing in as simple as LD.

Thanks!

I don't see any problem this is a start / stop latch in FBD does not take much more dropping the blocks & connecting them.

parky said:

I don't see any problem this is a start / stop latch in FBD does not take much more dropping the blocks & connecting them.

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Maybe I'm missing something, but how would that FBD ever work? If the output for Start Latch is ANDed with the Stop Button, how would that ever turn on the Start Latch? Maybe the Stop Button is NC, not NO?

defcon.klaxon said:

Maybe I'm missing something, but how would that FBD ever work? If the output for Start Latch is ANDed with the Stop Button, how would that ever turn on the Start Latch? Maybe the Stop Button is NC, not NO?

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Stop button is electrically wired as NC. So as long as the stop button is NOT pushed, the input will be true.

You should be able to do a “loop back” of the tag you want to seal in through an OR block with the start button. You’ll most likely need to use the “assume data available” marker or it won’t compile. I never actually done it because I usually will add a ladder routine for that type of stuff because I feel like it’s more intuitive for the next guy.

g.mccormick said:

Stop button is electrically wired as NC. So as long as the stop button is NOT pushed, the input will be true.

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That totally makes sense, thanks for the confirmation.

Seems like FBD is good for some work, but ladder is more intuitive for other things. When you see an NO or NC boolean, you immediately understand what's going on. Here, it doesn't seem as intuitive. Not rocket science but I still see a huge value in ladder for quickly understanding what's going on in stuff like this.

scameron81 said:

You should be able to do a “loop back” of the tag you want to seal in through an OR block with the start button. You’ll most likely need to use the “assume data available” marker or it won’t compile. I never actually done it because I usually will add a ladder routine for that type of stuff because I feel like it’s more intuitive for the next guy.

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Thanks for the response. Yeah as much as I hear people tell me LD is dead, FBD is where it's at, I still see a huge value in the simplicity and intuitiveness of LD for simplistic situations such as sealing in coils and what not. FBD is great for certain things but I still deal mostly in LD and still love it. Especially in Logix where you can export rungs to Excel and very quickly create new logic.

Glad somebody realised that normal convention is that Stop buttons are N/C physical contact to ensure a stop if pressed, Using a N/O means that loss of a wire on a Stop button would not stop when expected. I agree that in some circumstances ladder is easier to read when monitoring & for people who cut their teeth on ladder it makes sense. There is no difference as far as code (well not strictly true as some functions especially in ST create more code due to compatibility between platforms). In Mitsubishi even though set for FBD you can still use Ladder and I often do, not for my sake but for any plant maintenance engineer can monitor easily.

Here is some logic in ladder & FBD in the same block, I have added a couple of interlocks one N/C & one N/O. The beauty about FDB is in one and block you can add more contacts & invert them on the same block.

defcon.klaxon said:

Thanks for the response. Yeah as much as I hear people tell me LD is dead, FBD is where it's at, I still see a huge value in the simplicity and intuitiveness of LD for simplistic situations such as sealing in coils and what not. FBD is great for certain things but I still deal mostly in LD and still love it. Especially in Logix where you can export rungs to Excel and very quickly create new logic.

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I agree with the concept of ladder being easier to work with and understand for most electricians and techs. I've written some extensive programs, (ladder) as an example for air handlers and test cell temperature regulation and discovered that a FBD routine is real handy for scaling, especially in 5000. I first saw that a few years ago and after I understood the concept and the instructions, I would not use anything else for scaling. I will say that not all folks have the FBD option available in their license for Rockwell; I work in an auto assembly plant, and we're still using PLC 5 in a lot of places. My company's software for RS 5000 doesn't allow FBD--not enabled. That would be the only drawback I could see. We're shutting down this week for a re-tool and a lot of the new equipment has ControlLogix, however there's still a lot of PLC 5--a LOT. All ladder.