understanding ladder logic

[chanuajohnson]

Hi, can anyone help me, to understand some ladder logic....(Have the logic)

Anxiously waiting
Chanua

 

[Lancie1]

I will try to help you, and there are many others here who can help. What do you need?

 

[CroCop]

Where's your logic you want examined?

Do you want a sample?

How about this?

XIC I:0/0 OTE O:1/0
XIO I:0/1 OTE O:1/1

Translation:

If I:0/0 is off, O:0/0 is off.
If I:0/0 is on, O:0/0 is on.

If I:0/1 is off, O:1/1 is on.
If I:0/1 is on, O:1/1 is off.

 

[chanuajohnson]

Thanks
I have some ladder logic, that i am to understand. I am retrofitting a speed control system and to convert the ladder logic to function block diagrams.
However I cannot understand what the logic is doing.
I believe that when I do I can acheive much, however for some time I have been stuck.
the first few rungs of the logic, seems to initialize some registers.
I am to probably convert this logic to state flow diagrams as an intermediatry step before I can convert it to FBD's

Can you assist, or provide a method of which I can follow??

 

[chanuajohnson]

Here is a print screen of the first few rungs

I can send u a print screen of the first few rungs, see attached

 

[Lancie1]

Chanua,

Possibly you are expecting a light bulb to go on, and eureka! everything willl be clear. It is much harder than that. You have to study each rung until you understand the purpose of EACH instruction. If you don't know what the instruction is, then look it up and study some more.

Many programs do follow a routine. One of the first things done is to Initialize Variables (to some known value). FOr example, counter values may be reset to 0. These variables are numbers used during the program. Some variables may be set during the first scan only, while others need to be reset for each scan.

The middle of the program will consist of inputs and outputs. At the end, many programmers place auxiliary subroutines for things like data, alarm routines, and data storage.

The end of the program is simple --the END statement.

 

[chanuajohnson]

Hmmm, but my problem is the conversion, to the stateflow diagrams.
I guess as you say I need to take it in steps.

Is it that in the initialization phase this conversion to state flow disgrams is not needed?

Also i am confused as to how to figure out which outputs, in the logic refer to what hardwired logic. You know the labels and the actual equipment, How do I create a link in that sence.

And lastly do you know of any literature that can assist, as to a procedure in which to follow in order to accomplish my task

 

[Lancie1]

Chanua,

I am not familar with either of the PLCs, neither the original nor the one you are converting to. To help you with details, I would have to study the program, plus learn all the commands for the specific PLCs. Would it not be easier for you (a student whose job it is to learn) to do this, rather than me, an old guy with little time left for studying?

If you can break the problem down into small steps, and explain each step in detail, then you will get some help here. Of course, when you explain each step in detail, you will suddenly realize that you no longer need any help!

 

[chanuajohnson]

Thanks,
I guess i just collected a boofing. My aim was not for you to complete it for me at all. I have been working on this for approx. 2mths, not continously though, and was just concerned of my progress and wanted some assistance.

Thanks for the clarity of what i have to do, I have been studying it and some how I am not gaining much headway. I am not about to give up, but was just seeking assistance.

And I thank you for the assistance/ boofing.

Will keep at it.

 

[Lancie1]

State flow diagrams are not necessary for any part of a PLC program. There are many ways to write programs. For PLCs, ladder logic is common, but function blocks are also popular. I think your original program is ladder logic, is it not? You need a Programmer's Manual for that specific PLC program. This manual will explain in detail what each instruction does. Then you can figure out what the rungs are doing.

To make state flow diagrams, you do not actually need to know how the hardwired logic is connected, but it would help your overall understanding. Here again, you have to have user manuals for the PLC. You should be able to get them from your school, or from the PLC manufacturer's online site, if still in business.

 

[Lancie1]

What is this "boofing"? My only purpose is to help you help yourself. The only true help is not to do the work, but to encourage you to figure out how to do it for yourself. Only in that way will you learn anything useful.

Besides, as I said, I don't actually know what your program does! I would have to figure it out, just as you will.

 

[chanuajohnson]

Yes you are right the original program is ladder logic, I do have user manuals for the PLC.

My aim in determining the hard wired connections was, yes to aid in my understanding of the material and the system as a whole.

W.R.T the state-flow diagrams (SFD), I was thinking of converting the ladder logic to SFD so as to assist in the conversion to FDB,( not sure if this is a good or bad idea.)


PS. the PLC that the logic is to be converted to is TRICON, using TRISTATION developers workbench.

 

[Lancie1]

There's your key. Take the TRICON manual and look up the description of the first instruction on the first rung. What is it? (You did not actually attach your program previously). From your earlier post, I remember an Input switch that caused for or five numbers to be moved from one location to another in PLC memory. This is probably part of that "Initialization" step we talked about earlier.

If I were doing it, I would skip the middle conversion step. That seems just another complicating factor. As it is you will have to become familar with two programming languages. A third language will just make it more difficult.

Some previous PLC experience would be a big help here.

 

[chanuajohnson]

Hmmm No experience

I have attached it again, I understand where you are comin from with the incorporation of the third language, if not needed why do it.

I thank you for the advice, I have thought of working backwards using the TRICON however have decided to try it using the state flow diagrams initially.

I can now move on to that option, seeing that I have been stuck with the former.

Hmm and no, I have as much as zero PLC experience, but I have to start somewhere, huh. I guess this is as good a place as any, well i do hope it is...smile.

See attached..

 

[TConnolly]

I'm by no means a MELSEC expert but I have used some Mitsubishi Fx series PLCs, so I'll take a stab at it.

The ladder you posted clears some data registers and programatically determines how long the program scan time is in microseconds.

The output instruction on the first rung is a File Move. It is going to place the number 0 in 10 consecutive registers beginning at R996.

The output instruction on the next rung is a Pulse instruction. It will turn the bit M999 on for only one full scan of the PLC program when M9034 goes true.

The third rung first moves a Zero into register 996 and then turns off the bit at M995. This is triggerd by the single shot bit set up using the PLS instruction in the previous rung. Thus the second and third rungs function so that when M9034 goes true, R996 is set to 0 and M995 is turned off just one time.

As soon as this occurs, timer #210 starts in the next rung. This timer will count for 2500 counts. What I dont know is what the time base is for this timer. It might be milliseconds (2.5 seconds) or 10 milliseconds (25 seconds) or 100 milliseconds (250 seconds). You will have to look up the time base for timer 210 in the manual for the particular PLC model.


In the next rung, as long as the timer has not reached its preset of 2500, it increments R996 by 1. It will increment by one scan while the timer is timing, thus counting the number of times the program is executed during the timing period of the timer 210.

At the end of the time period bit M995 is turned on, and the number 25,000,000 is divided by the number of scans that was counted in the previous rung. The result is stored at R1000. This gives you the program execution time in microseconds, averaged out over the timing period.

The Mitsubishi manuals are available here:
http://www.meau.com/eprise/main/sites/public/DOWNLOADS/Manuals/default


edited to add: OK, now that I think about it, the timer at 210 has to be a millisecond timer for 2.5 seconds, because according to the comment the program time is in microseconds and the scan count is divided into 25 milliion. Mabye some of the real mitsubishi gurus here can confirm that.