You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old August 31st, 2005, 10:11 PM   #1
sweety616
Member
United States

sweety616 is offline
 
Join Date: Jul 2005
Location: New Jersey
Posts: 36
PLC Ladder Logic

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.
  Reply With Quote
Old August 31st, 2005, 11:04 PM   #2
TConnolly
Lifetime Supporting Member
United States

TConnolly is offline
 
TConnolly's Avatar
 
Join Date: Apr 2005
Location: Salt Lake City
Posts: 6,147
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.
  Reply With Quote
Old September 1st, 2005, 12:39 AM   #3
CaseyK
Member
United States

CaseyK is offline
 
CaseyK's Avatar
 
Join Date: Feb 2004
Location: In the cornfields, on the prarie, outside Chi-Town, a few miles beyond the boondocks.
Posts: 1,731
Lightbulb

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
  Reply With Quote
Old September 1st, 2005, 10:27 AM   #4
RussB
Lifetime Supporting Member
United States

RussB is offline
 
RussB's Avatar
 
Join Date: Oct 2003
Location: Michigan
Posts: 2,910
It is also important to know and understand the machine or process that you are controlling.
__________________
LEARN something today so you can TEACH something tomorrow.
DETAIL in your question promotes DETAIL in my answer.
Dominus Vobiscum <))>(
"Where is the wisdom that we have lost in knowledge?" T.S. Elliot


  Reply With Quote
Old September 1st, 2005, 01:20 PM   #5
kamenges
Member
United States

kamenges is offline
 
kamenges's Avatar
 
Join Date: Nov 2002
Location: Brillion, WI
Posts: 3,661
Quote:
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
  Reply With Quote
Old September 1st, 2005, 04:02 PM   #6
leitmotif
Member
United States

leitmotif is offline
 
Join Date: Nov 2004
Location: Seattle Wa. USA
Posts: 3,680
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
  Reply With Quote
Old September 1st, 2005, 09:28 PM   #7
sweety616
Member
United States

sweety616 is offline
 
Join Date: Jul 2005
Location: New Jersey
Posts: 36
Guys:
Thank you very much for your suggestions. I will come back with more questions as time passes
  Reply With Quote
Old September 2nd, 2005, 08:46 AM   #8
Tom Jenkins
Lifetime Supporting Member
United States

Tom Jenkins is offline
 
Tom Jenkins's Avatar
 
Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 5,908
Quote:
Originally Posted by sweety616
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.
  Reply With Quote
Old September 2nd, 2005, 08:53 AM   #9
mikedba
Member
United States

mikedba is offline
 
Join Date: May 2004
Posts: 10
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?
  Reply With Quote
Old September 3rd, 2005, 12:56 AM   #10
Leadfoot
Member
United States

Leadfoot is offline
 
Leadfoot's Avatar
 
Join Date: May 2005
Location: Red Stick, Cajun Land
Posts: 577
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?
  Reply With Quote
Old September 3rd, 2005, 07:11 AM   #11
jamesau
Lifetime Supporting Member
United States

jamesau is offline
 
Join Date: Mar 2004
Posts: 166
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.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
ladder logic program arlo f LIVE PLC Questions And Answers 50 August 4th, 2010 07:54 PM
CarWash Ladder Logic sicminds LIVE PLC Questions And Answers 30 November 14th, 2007 02:54 AM
Need help with ladder logic problem RickGilbert LIVE PLC Questions And Answers 10 June 5th, 2005 02:38 PM
Converting PLC ladder in Fuzzy Logic chico LIVE PLC Questions And Answers 5 October 17th, 2003 08:43 AM
Ladder Logic and Methane Detectors djinak LIVE PLC Questions And Answers 16 April 17th, 2003 02:49 AM


All times are GMT -5. The time now is 10:51 AM.


.