SLC 5/04 system without any comments

[plcnovel]

Hi There
Well is it possible to look at the ladder logic that is uploaded from the processor onsite without any comments and symbols and not only understand it but also add some more digital pressure and level switches. Also, when the program is not even written by you. Is there a way to upload the ladder from the processor with the comments and symbols on?

My system is

SLC 5/04
1 - 1746-IB16
2 - 1746-IB16
3 - 1746-OW16
4 - 1746-NI8
5 - 1746-NO4I
6 - 1746-NT8
7 0 1746-NT8

Thanks

 

[theDave2]

Comments and symbols are stored in the file on your computer, not in the plc.

 

[unsunghippie]

No good news

If you don't have a copy of the original, documented program, you are out of luck. You're best place to start is, hopefully, a set of electrical prints for the system. If you don't have that, maybe you've got an HMI attached where you can start to make sense of things. If you don't have either, I feel for you.

 

[Tom Jenkins]

Possible, yes.

If the logic is not too complicated, if you are familiar with the machine and process, and if you have wiring diagrams so you can identify the I/O, it is possible but not very simple to reverse engineer the logic.

Essentially you start with a single input, track it through writing your own comments and documentation for a best guess, and keep going until you reach an output.

You may also be able to work back from the outputs in some cases. For example, if you have a pilot light labeled "Alarm" anything that turns it on is going to be a fault. You work back from that to the input that initiates the fault condition.

If the logic is very complex you are going to have a very difficult time doing this.

 

[plcnovel]

Thanks for your reply
What if i just need to add couple of level switches and pressure switches, i don't think in this case i need to know a lot about the existing program. If i can manage to figure out the digital input configuration i think it would be possible for me to add two new digital inputs. However, it will be difficult for me to make those two new inputs (Pressure and level switch) as a part of unit shutdowns.

 

[tomalbright]

maybe, maybe not.

Unit shutdowns are usually long series contact runs with a single output.

Find something else that you know is a shutdown, find its IO, and then track it through the program. When you add yours, folllow the same scheme. Even lacking documentation such as drawings, it's possible to identify IO, and that goes a long way toward understanding what your program does. Even if you can't identify a particular address as per function, put a comment on it like "I think this is the water dingler". You can always correct it later, and as you fill in blanks you can see where you've been.

The sticky stuff is if the PLC is doing any alarm management. You would have to know what the setup is to match it for the new stuff, unless the customer agrees it's not important.

Cross-reference is your new best friend.

 

[Lancie1]

What if i just need to add couple of level switches and pressure switches, i don't think in this case i need to know a lot about the existing program.

First question: Who DOES have the original program? Should not you talk to that person(s) first? Maybe there is a reason that you don't have a copy of the comments and symbols. If you don't know a lot about the existing program, then find someone who does.

 

[plchacker]

First question: Who DOES have the original program? Should not you talk to that person(s) first? Maybe there is a reason that you don't have a copy of the comments and symbols. If you don't know a lot about the existing program, then find someone who does.

I agree with Lancie1. If you insist and if your level of knowledge of that machine is very thorough, you might be able to make the changes you like. Yes it can be done, but be prepared to spend a lot of time.



I recently tried to work on a De-Min plant without documentation. It looked simple enough, until I opened the program the first time. I shut it down within ten minutes, and dug around until I was able to find the original program/documentation. Things went much, much better with documentation.