From: Mike N. 

On Sun, 10 Dec 2006 20:23:32 -0800, Ellen K.  wrote:

>OK, you two, I am intrigued now:  What is a ladder diagram or ladder
>logic?

   It is a programming methodology for industrial equipment control that
uses a visual circuit representation of program logic: relay coils and
contacts that can be arranged in AND, OR, NOT,  XOR, etc configurations.

   Unlike so many other "let's give them icons and let any dummy write
programs" software models, this one works.    But it needs a programming
methodology on top of it for anything more than trivial applications.
Otherwise complex ladder logic quickly degerates into spaghetti code.

   The reliability of these applications (PLC) is generally many orders of
magnitude above the PC.    Buyers actually care about reliability and
quickly throw any vendor out on their can if they have flakey products. Not
so very long ago, controllers with 32K and 128K bytes of ladder logic cost
$5K to $10K, even while PCs were getting into the megabytes of RAM.

   Vendors created ladder logic programming software to run on DOS with
640K by operating from a database.  Screen display came right of uncached
database qeries, so it was often very slow.

http://en.wikipedia.org/wiki/Ladder_logic

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