The rapid advances in performance and miniaturisation in microtechnology are constantly opening up new markets for the programmable logic controller (PLC). Specially designed controller hardware or PC-based controllers, extended by hardware and software with real-time capability, now control highly complex automation processes. This has been extended by the new subject of "safe- related controllers", aimed at preventing injury by machines during the production process. The different types of PLC cover a wide task spectrum - ranging from small network node computers and distributed compact units right up to modular, fau- tolerant, high-performance PLCs. They differ in performance characteristics such as processing speed, networking ability or the selection of I/O modules they support. Throughout this book, the term PLC is used to refer to the technology as a whole, both hardware and software, and not merely to the hardware architecture. The IEC61131 programming languages can be used for programming classical PLCs, embedded controllers, industrial PCs and even standard PCs, if suitable hardware (e.g. fieldbus board) for connecting sensors and actors is available.