Ladder logic without symbols

Hi Guys,

I have two's question, please share:

1)Is there a best way to troubleshooting a ladder logic (RSLogix500/5) that has not symbols and descriptions.
Many customers or user doesn't have offline file in the diskette (because already old, damage, etc).So, when they are upload their program from PLC, the symbols doesn't appear.
Beside that, without symbol and description, I think is it difficult to know their machine or processes worked.
Any comment?

2) How to make people can be familiar or trained their brain to following logics on ladder logic.

Regards,

Deer

Deer,

Trouble shooting without annotation can be a pain, It just takes a bit of time and effort.

The way I go about this is as follows:

I first get hold of a copy of the electical schematics showing all the Inputs and Outputs associated with the PLC in question. Having got this information I will then nickname all the I/O's within the program. Then I would make a list of all flags used within the program and attempt to nickname them up. It should then be possible to work out what each rung does and annotate as required.

This is a pain in the arse to do, but having taken the time and effort once you should not have to do it again.

Deer wrote: 2) How to make people can be familiar or trained their brain to following logics on ladder logic.

Click to expand...

I think I know what you mean, I believe any electrician who can read electrical schematics should be able to read and understand straight forward ladder logic. After all, what is the difference between the two?

Good luck

Paul

PLUCAS said...
"I believe any electrician who can read electrical schematics should be able to read and understand straight forward ladder logic. After all, what is the difference between the two?"



To which I say... I can't believe you said that.

Terry

Perhaps you could explain to me why you can't believe I said that!

Then maybe I could defend myself.

I learnt to read ladder by thinking of it as trouble shooting the old fashion way, using a schematic and a multi-meter and measuring the voltage at various point of the circuit.

On an electrical schematic, you might have a couple of contacts, etc controlling a relay.
What is the difference between that and a couple of inputs (etc)controlling an output in ladder.

If I could work out how to post a couple of images on here I would, just to show you what I mean.

I hope this goes some way to explain to you why I said what I said, But I would be interested to hear your way of thinking.

Paul

"I believe any electrician who can read electrical schematics should be able to read and understand straight forward ladder logic. After all, what is the difference between the two?"

Click to expand...

I think it has something to do with all the flashing lights on the plc, it mesmerizes electricians and maintenance men.

In all honesty a large majority of electricians have no need to read ladder/machine schematics because they never deal with control work of any kind. Their primary duties are to run conduit, wires ...ie distribution. I would state that an industrial maintenance electrician
"should" be familiar with ladder/machine schematics and learn (and/or trained) how plc's interact even if not how to actually use the program to troubleshoot. In all reality though this is not always the case, employers want people to know these things when they hire them and training seems to be (in many cases) not something companies deem necessary. The employee is expected to know/learn on their own.

As for troubleshooting I agree with PLucas on the use of the schematic if available. With a schematic a knowledgeable/trained person may be able to determine the problem without the need to look at the program. I also believe a documented copy of the program/code can make it easier to troubleshoot. I always make copies of programs and reverse engineer/document them when I dont have or cant get the code from the manufacturer etc.

I am going to tell a story now, I took a temp position at a Dairy processing plant that was looking for plc/control techs, I was placed at top level (F1) for their maintenance people but I had to work nights. The night shift supervisor was a retired Marine that took a 2 year tech school course while working as a PM mechanic (F3), after passing the maintenance course he was made F2, two weeks later he was the supervisor. NO prior experience in this field at all. NOW comes the machine troubleshooting story. The dairy had a line that brought milk cases thru a washer and then fed them to be loaded with product..ie milk. This particular line was running half gallon containers, the containers would be positioned then dropped into the case then sent to freezer/cooler. 3 shifts worked on this case packer, changed every pneumatic valve etc on it. I was new but the night tech called me over and I checked the inputs/outputs and determined one output led was coming on BUT there was no power coming from the output. I dont remember the specific model (it was an AB) but it was one you could exchange the EEPROM in and retain the program. They had a spare. The supervisor decided I was wrong and AGAIN changed every part on the machine, they worked on it till 5 AM changing parts. Then they asked me "Can you exchange the plc?" I said yes and pulled the old out, moved the prom over and installed the new. It was up and running in less than 30 minutes from the time they said ok you can do it.

My reason for telling this story? People do not always possess the knowledge you think they should have. Companies dont always hire or train people properly for the positions they fill.

I refuse to work under anyone dumber than I so I didnt work under that supervisor another night.

As for helping a company troubleshoot it takes a few key factors:
1. People or at least one person willing to learn. On his/her own if necessary.
2. Companies willing to provide some form of training.
3. Management/Supervision that is knowledgeable.
Its rare to find one of the above let alone all 3.

As an after thought, many maintenance/electricians etc spend years being good at what they do, then are expected to KNOW anything NEW that comes along and how to deal with it. I know many people and I will say especially engineers do not understand WHY a maintenance man doesnt understand something or learn about it. You took your route to learn what you did because of desire and intelligence, not all people have the desire or the intelligence of others. People are different, that said that maintenance mechanic, electrician etc may be GOOD at his/her job but just doesnt have the knowledge YOU think they should have.

Ron and Terry,

Perhaps in my original reply I should have specified 'Maintenence electrician' instead of saying just 'electrician', my appologies for not clarifying this point. I assumed that everybody would know that I was talking about maintenance electricians. I call plain electricians (those that install socket outlets etc) 'house bashers', but thats another story.

Ron

You said

I refuse to work under anyone dumber than I so I didnt work under that supervisor another night.

Click to expand...

I am interested as to what your employers said to that, did they except that they had promoted a person who did not have the relevant knowledge to supervise the technicians?

Unfortunately, companies do not always promote people because of their knowledge of the systems that they have to work on, when my company promoted a guy to supervisor after only 3 months service (he shall remain nameless) , all the technicians expressed concern that he knew very little about our cranes etc. The management reply was that he was dynamic and in his interview he had impressed the senior managers with his ideas and beliefs on how the shift should be supervised. That seems to happen a lot! But that something we have to live with and just get on and do our jobs to the best of our abilities.

Paul

Terry said
Perhaps you could explain to me why you can't believe I said that!
Coming from a wiring background may be I could help explain.

I started out as electrician (wired many machines) and a control panel wireman. Yes following a straight forward PLC program is very similar to "old fashion" relay logic schematics but with the relay logic most of the time you saw a large part of the circuit all at once. Meaning you did not only 4 or 5 rungs at time on screen. Also with a PLC you could tend use more contacts than if everything was hard wired. Trust me it is very intimidating going from a tool belt to a laptop. Coming from the shop ( I really do not mean to offend) you sometimes encounter an arrogance from some programmers, but definitely not all, that they are performing magic. The hardest part was being allowed the time to play until that moment when things started falling into place and all that math you ignored for years gets get to front of the brain.

Thanks for letting me vent.
JIM
GO JETS

Thank you Jim

You understood what I was trying to say about PLC's and good old fashion relay logic. With a good background in relay logic, you should be able to read and understand ladder. (that is my opinion)

Just for the record Jim, I did not see a PLC until after 15 years in the 'shop' (including apprentiship),I first worked with PLC's when I started my current job (5 years ago)and I learnt to read ladder using the method I stated in a previous reply.

I do not class myself as a programmer. I am a maintenance technician who happens to do programming as well, not the other way round.

Paul

First...

JimTech67 said that "Terry said...

Perhaps you could explain to me why you can't believe I said that!
Coming from a wiring background may be I could help explain. "

I realize that this is only a case of confused cut-n-paste... but I just wanted to clarify that I did not say that.

The original statement from PLUCAS was...

"I believe any electrician who can read electrical schematics should be able to read and understand straight forward ladder logic. After all, what is the difference between the two?"

To which I then said... "I can't believe you said that."

My unspoken point was...

To infer that...

...knowing how to read an Electrical Schematic implies the ability to read PLC Ladder Logic,...

is like saying...

...knowing how to add and subtract implies knowing Calculus!
--or--
...knowing Newtonian Physics implies knowing Einsteinian Physics!

The key difference in both of those comparisons is "time"!

Certainly, both Calculus and Einsteinian Physics can describe events without being subjected to "time".

But, if you need to include "normal" time effects through Euclidian Space, the kind we normally experience, Addition and Subtraction doesn't cut it.

Likewise, if you need to include the "serious" time effects in Physics, Newton doesn't cut it.

The point is, a line of so-called "simple-code" can have some effect on a condition (here and now, at this time in the scan) and later in the same scan, some other line of "simple-code" can have a different effect on that same condition.

For an electrician to be able to grasp what's "really" going on in a PLC program, he has to come to terms with the fact that a single line of code does not necessarily make an output!

Actually, it is quite typical for a non-programming electrician to become very aware, very early on, that there is more than meets the right-now-eye when looking at a line of code!

The mainstay of PLC's is their ability to use and maintain Internal Control Relays. These Internal Control Relays are, or at least, can be, used somewhat differently then they are used in a real Electrical Schematic.

The difference shows up in the "timing" within the "scan". The placement of a Control Relay in a Real Electrical Schematic is not subject to "scan" time. In a PLC Ladder, the relative placement can mean the difference between a system that works and a system that doesn't work.

It's a case of a "Real-Time System" vs. a "Scan-Time System".

This is what confuses inexperienced programmers and electricians... even if they know how to read a schematic.

To imply that PLC Programming is as simple as reading a schematic is wrong. Otherwise, wouldn't all schematic-reading-electricians be PLC Programmers?

Gentlemen,
Sometimes we do not always programme as we would draw a schematic, we may not always use an output to finish the rung afterall there is more than one way to skin a cat ( for want of a better saying ).For example,if the machine operation is sequencial you may decide to use "step" , the first rung starting with a first pass and finishing with a MOV each rung after this starts with an EQU and finishes with a MOV and just to to really **** people off you use indexed addressing and control the outputs by using an MVM and set the required data file bits.
Also , most electricians cannot read schematic diagrams in fact I consider most of them to be thick. By the way I am a registered electrician with advance trade certificate.In New Zealand, up to a few years ago, to become an electrician one served a 4 year (8000 hrs ) appreticeship , attend night tech , do block courses , and pass your exams to become a registered electrician

Terry

You said

To imply that PLC Programming is as simple as reading a schematic is wrong. Otherwise, wouldn't all schematic-reading-electricians be PLC Programmers?

Click to expand...

That is not what I implied, I did not mention programing, I said that electritians (maintenance) should be able to read and understand simple ladder logic if they can read schematics. I am aware of the complexities of the PLC scan and how it can effect programs, but if I was to show a maintenance electrician a rung of simple ladder logic I would expect him to be able to tell me what needs to happen to energise the coil.

I hope that clears this matter up Terry, it is just a matter of opinion and mine differs from yours. We could 'debate' this till the cows come home, I think that we will just have to agree to disagree on this one. If you ever visit this country, maybe we could continue this discussion over a beer or two, or three, or four........

Paul

Hi all,

Schematics diagram ? Yeaahh...I think it must be consider to help us if the ladder logic doesn't have documentation.
Anyway, thanks for advise and your opinion so this forum to be warm..


Regards,

Deer

Deer, all tangents aside what you propose is difficult. Troubleshooting is not something you can teach, you can write guidelines to assist people in sequences to use and what to look for
BUT
The key word above was "people". The ability to reverse engineer, troubleshoot, diagnose, read schematics, read RLL, read program code etc etc is dependent on the abilities of the personnel at the facility.

In the case of NOT having documented copies of the code then maybe you can work with them on obtaing the code (when possible) and documenting/describing again. This may be a time consuming proposition.

You may also discuss with the companies the option of adding more code (if memory allows ) for alarms etc. This is an excellent way to minimize downtime and troubleshooting.

Sorry Terry my bad

Jim

GO JETS

I have to do this all the time. Diagnose a fault on a PLC controlled machine with no documentation, no comments, no nothing. And usually a machine I have never seen before either.

It is a long process and you have to document what you can as you go along. First you put the program in monitor mode, then.......

Press buttons, actuate limits, start motors and actuate valves etc.

As the comments and statements build up in the program the fog starts to clear.

It is worse if there is an HMI that you do not have the software for. The PLC program is littered with unexplained internal bits that are controlled from the HMI. And registers with data in them that you are not sure how it got there.

It is however ultimately satisfying to crack the problem when it seemed as if everything was against you.