Does anyone know which PLC is the most commanly used in industry? I am wanting to get some training done in PLCs but want training the the most commonly used ones. Someone told me that the AB SLC500 is widely used. Is that true?
There is no single item called a PLC therefore there can be no single answer. This is no different than asking "what is the most common handtool used in industry".
You will get lots of personal opinions and that is all. Without a doubt the most common name in PLCs in the US and Canada(?) is AB, but they make very many different models. They are not the market leader in each model, in each industry, in each country.
PLCs are tools used to automate industrial control systems. In North America the most common method of describing control circuits is by using Ladder Diagrams. You need to make sure that you learn about control circuit logic and not just the command set of a particular manufacturer's programming package. Once you know ladder diagrams and logic systems all you need to do is learn the unique features of each PLC model/brand you work on.