For those new to the world of PLCs, getting a handle on what they
are, what they do and how they do it can be somewhat intimidating at first.
The same could also be said of computers, and of course that's exactly what
a PLC is. Well almost!
In fact, there's a bit more to the average PLC than just a computer. There's
no doubt that a computer is the heart of the beast, but then there's also all
those interface modules (I/O) that allow the beast's computer to monitor and/or
manipulate endless types of electrical devices and equipment.
Then to make matters even more interesting, most PLCs aren't exactly
programmed like other computers. PLCs were designed from the onset to be
installed, wired, commissioned and maintained by electrical personnel rather
than your average computer science grad. As a direct result of this electrical
influence, the PLC programming language of choice evolved to become
"Ladder Logic", a programming language that looks more like a wiring diagram
than the structured lists of computer directives that are normally associated
with languages such as C++ or Visual Basic.
So what does this all mean? Well basically what we end up with is really
three significant areas of study when it comes to PLCs: (1) Computers,
(2) I/O (interface) and (3) Programming. You don't have to be
an expert in all three to work with PLCs, but you must have an understanding
of all 3 before you can claim that you know how PLCs work.
Assess Your Needs!
Obviously only you can assess how much you already know and which method works
best for you when learning new concepts. Many people need the contact and structure
associated with a formal classroom environment while others can do quite well with
a book and a little hands on to ingrain their new knowledge. Be practical, and most
importantly be honest with yourself. How badly do you want to learn PLCs?
How much effort, time and money are you willing or can afford to expend?
Ask around, snoop around, shop around, and then make a plan and stick to it.
Formal PLC Courses
For many this is the only way to go and for most it's the best and quickest
Method of learning the fundamentals PLCs. Course's are available in many formats,
varied lengths and from many sources. Almost all have value, but the best are
typically those which provide a 30 to 60 hour combination of instruction and
Hands-on labs. Major PLC manufacturers, offer such programs, usually as single week
Full time offerings; typically to small groups and often charge $1200US plus but
this usually includes lunches and a full set of manuals. Many colleges and training
Companies have directly copied the curriculum of these courses and offer these
Knock-off programs to both day and night school students. Prices can get down to
The $200-$300US range thanks to your taxes, but may often mean larger classes,
poorer equipment sometimes, and definitely no free lunches.
If you're lucky and do a little shopping around you may be able to locate
a high quality PLC course, taught at a relaxed pace and utilizing the latest in
equipment at a bargain price right in your hometown. If you have a particular
manufacturer of PLC in mind then this will of course effect your choice, but
otherwise settle for any of the "Majors" and make sure you go for the Level I
program. If you skip to an advanced level you will miss all the computer and
hardware aspects which are so crucial to the understanding of PLCs.
Approaching PLC Training on Your Own!
Whether a PLC course is out of the question; you require a little more
help with the theory, or you're just in need of more hands-on practice than
is available in the course you're taking; then you have a couple of points to
consider. There are two major learning components associated with any PLC
training program, and from a training point of view they dictate the use of
different skill sets and student activities.
First, there is the requirement to learn about the
components of PLC systems, why the various components are there and how they work.
This is the theory aspect of PLC training and is normally presented in a lecture
format in the classroom. This material is best re-enforced with online or CD
based tutorials, printed handouts, and/or a good PLC textbook.
Secondly, there is the
hands-on aspect of the training. This is where the student is required to
create, enter and debug PLC programs using their newly acquired knowledge of the
PLC and it's programming instructions. In the past this aspect of the training
always made use of actual PLCs which where wired to an assortment of switches,
lights and displays and then programmed by the student via a programming terminal
or a desktop computer running the PLC manufactures programming software.
We couldn't achieve the realism of being wired to an actual industrial process
such as a conveyor, but with a little imagination we made do. Today it is now
much more common to use software graphics to simulate the controlled equipment
and lately a number of packages can now also simulate the PLC as well.
Professor Bill's Recommended Course of Action:
I may well be biased with these suggestions and of course I reserve
the right to change my mind in the future, but here goes!
- Take a formal PLC course if possible. Unless you have access to actual
PLCs at work you're going to be at a disadvantage trying to digest all
the hardware details from just a book. For many there's nothing like
having the physical product in front of them to re-enforce the details.
- Purchase a good PLC textbook such as:
Each of these books is a good dollar value, and you essentially get what you pay
for. There are certainly other good ones to be found, and almost any text is
better than having none at all.
- Consider the Purchase of a CD based Tutorial Package. This is a bit more
expensive than a book, but for many the interactivity of a good computer based
training package (CBT) helps to keep one's attention focused. Take a look at
which is available from
for just under $160, but this price also includes a copy of the CD/Key Edition
- Carefully work through the hardware and operations sections of the book
or CD you have purchased and complete each of the end
of chapter exercises. If you take your time and are thorough, you should
end up with a pretty good grasp of the fundamentals of PLC operations,
hardware and wiring upon completion of this task.
- While reading the above chapters, go to both
The PLC Tutor and the
Siemens PLC sites and work through the appropriate tutorials they
have available. If you don't have a computer with Internet access at home,
then by all means this is the time to get one!
- Once you get to the programming aspect of your book, download our
PSIM program(s) and print
out the documentation and student exercises. Proceed with the student
exercises in the order in which they are listed. When you start an exercise,
reference the appropriate chapter in your textbook and read it thoroughly.
- For documentation of RSLogix commands and instructions supported by
LogixPro. Refer to the
Programming Command Guide site for examples and details.
- While you are working on this course of action be sure to take time to
investigate the PLC and Automation sites I have listed on my Links page,
visit the Q&A boards at
often and don't hesitate to ask a question while you're there. There are some
really top-notch people who regularly drop by and they are more than willing to
help others out.
What can you expect in the End?
Based on teaching PLCs for 15 years, there is one thing that I can guarantee
that you will find out about PLCs and particularly Ladder Logic programming.
The simple programs and instructions you start your training with are by far
the hardest to grasp and clearly understand. As you advance you will be
amazed at how much easier it all seems to get, It's just the nature of
the Ladder Logic beast! So take heart, take it slow and don't skip to
those advanced instructions until you know the relay logic ones off by
heart. Yes they really are That Important!
If you fully accomplish all that I have set out for you above, you should
now have an excellent grasp of the fundamentals of PLCs. That doesn't mean you
are ready for a job as a lead programmer or that there isn't a lot more to learn.
It does mean however, that you've gotten through the worst and you've proven
that you're up to the challenge, and prepared to move on.