Hello, I have worked a long time with industrial automation and I can give you a north.
Communication with PLCs can be done using various means, one of them is with OPC protocol (OLE for Process Control), most PLCs (Siemens, Rockwell etc) have this protocol, but there are other interfaces such as RS232. OPC is a protocol that facilitates the capture of data, usually the manufacturer provides a server controller that interfaces between your application and the hardware. In the Rockwell CLPs of the Contrologix family, this interface is RSLinx for CLPs In this "server controller" you map all the addresses of the PLCs in your network to add the bits of each PLC that you want to "share." After this configuration, you must have a component for your programming language , in my case I used Delphi with the Kassl OPC Client component, but there is also the .NET version. components you configure the addresses of the PLCs and the bits you want to capture.
The CLP can store some information, but it is limited, because the hardware has little storage capacity, and is programmed to perform a task. Then it provides you with the information in real time, you simply capture this information and structure in your SQL database and generate history if it is the case.
There is a very interesting free software project, the ScadaBR . This software has all the components necessary for you to start developing your PLC monitoring system.
There is not much information on industrial automation even on the internet, it is a very restricted market, but it has been worse in the past.
