PLCs (Programmable Ladder-logic Controllers) are what industry uses to automate their factories. These versatile controllers are used to automate equipment from rocket fuel plants to bakery ovens (and in our case AQUARIUMS!)

Ladder-logic is a language that looks (and acts) like an electrical ladder diagram. I've programmed in more languages than I want to think about (including assembler) and find that Ladder-logic is not only a self-documenting-graphical language, but the fastest way to get one-of-a-kind applications up and going. We call Ladder-logic a self-documenting language because when you are finished, you can print out a copy that your plant electrician can read and understand! Why? Because it uses the same graphical representation that electricians call ladder diagrams with the same types of symbols they have been looking at for generations.

What we have with a PLC is an 'automation controller' (not a computer optimized for general-purpose duties like text editing and rigged up to run a key bottleneck in a production line). While PCs are great for programming the PLCs themselves they just aren't reliable enough to trust with real world production applications. While it may not be a big deal to get a 'FATAL EXCEPTION ERROR' while writing a letter, it could be a very big deal if you were say, running a high speed web press - or monitoring safety interlocks of a 20,000 ton impact punch. PLCs have simple and robust operating systems. If a PC is being used as a 'User interface' (a user interface is used to display information and to make adjustments and often runs under Windows) and it gets a software hiccup - while you are cursing Mr. Gates during reboot, the PLC keeps everything working safely and smoothly.

What makes the use of PLCs a revolution today is that the price has gone down to the point that it is often as cheap (if not cheaper) than what industrial OEMs have been offering as their custom extra feature package! This means not only can you upgrade your equipment to do the new things the OEM is offering, but you can have it do even more and while you are at it, have it do it your way!

• Controlling cardboard bailers, greenhouse temperatures, and HVAC
• Controlling equipment that makes garage doors, power shedding, and plastic injection machines
• Running commercial washing machines, and automatic cattle feeders
• Safety interlocks, water-plants, and conveyer lines

Building cars would be quite different without PLCs. PLCs can talk to your PC through a 'software protocol' or to each other; sharing data or receiving data that tells it what to do! Networking is natural to them -- PLCs have been talking to each other before you even had a network card!

A 'brick' PLC has a block shaped case (hence a 'brick') with terminal strips on it. They can be panel or DIN rail mounted but come in a model-number-determined set of I/O capabilities. A 'Rack' has one or more 'card racks' that individual circuit cards slide into so you can pick and choose your I/O to whatever number and/or type you need.

Yes, the PLC has an RS232 connection and with Software options would allow you to read and write to any of the registers. Also the User Interface with a LCD display screen. There is also high speed networking capabilities available.

For a Really good Tutorial on PLC's visit http://www.plcs.net/contents.shtml