any tips 4 programmer to learn PLC?

Hi All,

I am an accounting software developer (foxpro) and Master in CompSci - who is plan to take a class in PLC (4day class).
Frankly, I was lost when saw the ladder program....
Please give me tips to learn the PLC and which PLC should I learn (the most popular/powerful...) Do I need a different strategic way of learning this PLC ?????

thanks so much.

Joe2000

At the top of this page is a "Learn PLCs" link that is one of the best very basic courses on PLCs

For more advanced topics, see the link below for a detailed textbook.

http://claymore.engineer.gvsu.edu/~jackh/books/plcs/

For PLC work you need an understanding of basic electrics, and an understanding of digital electronics at the AND/OR gate level is also good.
Ladder logic is a compiled high level language that will simulate a control circuit put together with relays.
In other words, it emulates a hardwired relay circuit. The reason this method was chosen was that the controls a PLC was designed to replace were previously assembled in hardware out of relays, and the intended customers for the PLC were already experts at drawing up these kind of circuits.
In addition to ladder, PLCs also use the following languages:
-Instruction List, which is similar to assembly
-Functional Block Diagram, which uses AND and OR gates
-Structured Text, which is similar to Pascal
-Sequential Function Charts, which is a representation of a flow chart.

Not all PLCs are programmed in all the above languages, but all PLCs that I know of can be programmed in Ladder.
Every manufacturer's implementation of Ladder is different.

Its a good job if you like machines.

Hello Joe.
One of the way you can learn is by this very website. It teaches PLC at the very basic level.
I also found an article which you can view to better understand ladder logic. here's the link:
http://en.wikipedia.org/wiki/Ladder_programming_language
And I always post this link for people who wants more information on the PLC: mikroelektronika

hope this helps.

regards
Sherine T.

Your biggest challenge is not going to be the ladder logic. It is going to be learning the functions of the machinery and process you are controlling, and understanding the electrical principals of the instruments and devices connected to the PLC. Get some introductory industrial electricians handbooks to get a grip on this.

One thing which seems to trip up lots of people coming from a computer science background is the concept of 'program scan'.

You must understand that the PLC executes ladder logic one rung after another*. Think of it as each rung being a self-contained process or task which gets the full attention of the processor while the rung is being executed. During this time no other rungs matter. When the rung concludes, the status of outputs and/or data table values is updated and the next rung becomes 'active'.

The machine does this constantly - when the last rung is reached control jumps back to the first rung and the process repeats over and over.

* Excluding conditional jumps/subroutine calls and interrupts.