PLC Ladder Logic

sweety616

Member
Join Date
Jul 2005
Location
New Jersey
Posts
36
I was hired as a Controls Engineer. I have some basic understanding of PLCs but definitely not an expert. I was given the electrical schematics and ladder logic of a machine. I am not able to follow the ladder logic. Is it necessary to have thorough understanding of the process before I proceed to understand the ladder logic? What is the best way to start. Any suggestions on how to get better handle of the subject is appreciated.
 
Yes, absolutely.

I suggest you take it very seriously. Peoples lives are litterally in the hands of the controls engineer. Check out the tutorial on this site, and get your employer to spring for some factory provided training on the PLCs you use.

Don't take it lightly. We recenlty had a thread here about someone getting killed because a controls engineer didn't do what he was supposed to do.
 
I had extensive experience drawing schematics when I started programming PLC's.

Ladder Logic to me looks like a schematic, with some small changes.

If you understand electric theory and blueprint reading, you shouldn't have too much trouble.

Some guys put a lot of fancy stupg in theier logic, which may or may not be neccessary. Don't get too fancy at first.

Staight Ladder Logic is just switches (or relay contacts) and relay coils, or in LL terms, Inputs and Outputs, with a few timers thrown in for good measure once in a while.

If you haven't yet, why not go through the Tutorials in the "Learn PLCs" button at the top of the page.

regards.....casey
 
It is also important to know and understand the machine or process that you are controlling.
 
Originally posted by sweety616:

Is it necessary to have thorough understanding of the process before I proceed to understand the ladder logic?

At the very least you need to have one or the other; you either must be able to understand software methods very well or you need to understand the process. Without one of these you won't have much of a chance understanding either the process or the code.

Your best bet is to understand the process first. The best you can do from looking at the plc logic is find out someone else's (the original programmer) idea of what the process is. This may or may not be correct. From your standpoint, understanding the process will allow you to look at the existing logic and understand why it was coded that way. Any sections where you ultimately say 'That just doesn't make sense' are areas where your picture of the process differs from the process understaning of the original programmer.

When you are writing a plc program from scratch the only option is to understand the process first. So ultimately this is always the way I approach it.

Keith
 
Originally posted by sweety616:

Is it necessary to have thorough understanding of the process before I proceed to understand the ladder logic?

Look at it this way
IF you don't know what the machine or process is supposed to do

HOW can you make it do what it is supposed to?

Maybe another way of looking at it is
IF you are going across town to point B how are you gonna get there if you don't know what roads to use? ASSUME it is NOT Rome to which all roads (supposedly) lead.

THe bottom line is this (and I learned this as a submrine electrician) you gotta know what the machine is SUPPOSED to do before you can figure out what is wrong (or IF there IS anything wrong).

Dan Bentler
 
sweety616 said:
Is it necessary to have thorough understanding of the process before I proceed to understand the ladder logic?

Let me put it this way - when I started my business almost twenty years ago we hired a "programmer". I got rid of him because I decided it was a lot faster and easier to teach myself programming than it was to teach the bit jockey how the process and mechanical equipment worked. It is one decision I've never regretted.
 
1756-hsc

We have a 1756 HSC that the Z input LED does not flash. A and B will flash, and if you move the A to the Z the Z will flash. The A will flash with the Z wired to it, but you can never get the Z marker pulse to work on the Z input. Any suggestions?
 
The Z pulse is probably passing by to fast to get full illumination on the bulb. There fore you cannot see it. The A pulse can also be WIDER than the Z pulse too. I suggest you scope the pulses and the bulb and see if the bulb is getting the ON condidition.

BTW, Does the process the encoder feeds work correctly?
 
Some Considerations...

Some suggestions to help you on your way:

1) Try to find a written 'Operating Manual' or general project documents related to the machine. This will often have a general machine description or 'Theory of Operation'. If not available onsite, a friendly call to the machine's fabricator will often be very useful. If not abvailable, Google the machine name or description and see what happens; you might get lucky. Also, sit down with a knowledgeable operator or maintenance person to interview for an operating description... be thankful and humble.

2) You have the schematics, so I assume you'll have a list and description of the I/O (inputs/outputs connected to the plc); this is very important info. Understand what each sensor and actuator actually do and where it's situated on the machine.

3) Get manuals of the specific PLC you're using, particularly the instruction set, data types, and hardware manuals. Hopefully, there're online manuals.

Good luck.
 

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