PLC is actually an industrial microcontroller system where you have hardware and software specifically adapted to industrial environment. Block schema with typical components which PLC consists of is found in the following picture.PLC Programming Special attention needs to be given to input and output, because in these blocks you find protection needed in isolating a CPU blocks from damaging influences that industrial environment can bring to a CPU via input lines. Program unit is usually a computer used for writing a program.
The PLC program not only provides control of all the motors, switches, gates and valves on the machine, but provides communications to the operator. Operator communication is often provided by an HMI screen. Although the HMI has a separate program, the diagnostics and alarming originates in the PLC.PLC Programming

A plant reporting system might also be collecting information from the PLC. Production counts and operating efficiency must be processed by the program to insure accurate information is fed to the data collection network.

PLC Programming Services The PLC must be programmed to work seamlessly with safety hardware defined in the control schematic. There are many tradeoffs between hardware and software solutions. It takes an experienced engineer to make sure the final solution is both a safe and efficient one.