The PLC, is a special purpose computer with no display, keyboard, printer, hard drive, but it is still a can be represented as a computer. PLC was a replacement for panels of relays, devices that turn on and off. Relays also fail far more than computer components, so that relay logic required more downtime to keep it running than newer PLC-based controls and require a lot of electricity, make lots of heat and soot, and take up lots of space.

An installed PLC is deceptively simple in appearance. It has a CPU module and input/output devices, referred to as I/O. The CPU communicates with the I/O, so in most systems they share a backplane that physically holds them in place and electronically connects them. The I/O modules can be separated from the CPU and connected with data cables. Hence, the PLC is not limited to a single cabinet or even the same building. Since the PLC is a computer, there is no need to limit it to only digital inputs and outputs. Over the years, the manufacturers’ added analog and several inputs and outputs. To make these numerical devices useful, they included calculation capability in the programming as well, so that we can now calculate, for instance, Statistical Process Control (SPC) values and set Proportional Integral Derivative (PID) controllers directly into the PLC program see figure. Since this is called a programmable logic controller, we need to program it. Most PLCs were programmed using an application installed on a standard desktop or laptop PC. The communication with the PLC was by using the Ethernet or a proprietary communication bus, depending on the manufacturer. Most manufacturers seem to have settled on Ethernet and/or USB in last past years. Although most of the manufacturers claim some form of Ladder Logic, the specifics of that language are different for each manufacturer, including different capabilities, different ways of doing the same thing, and even differences in the order in which the CPU processes the various pieces of logic. One attempt to bring some order and inter- operability to this chaos is the IEC 61131 standard from the American National Standards Institute (ANSI).”

The S7-300 and S7-400 PLCs are utilized in increasingly perplexing applications that help a more noteworthy number of I/O focuses. Both PLCs are secluded and expandable. The force supply and I/O comprise of independent modules associated with the CPU. Picking either the S7-300 or S7-400 relies upon the multifaceted nature of the undertaking and conceivable future extension. Your Siemens deals delegate can give you extra data on any of the Siemens PLCs. Figure clarify an
example of PLC S7-300.