It has used it for over than 20 years and some of you did the similar. Modicon 484 had 8 sequencers of 32 steps each on 22 years ago. Using it properly it has employed a 64-step state mechanism. On Omron PLC it has used the "4 to 16 decoder" control and currently with Mitsubishi PLCs it is using the "DCODE" control to do the same. On the one of State Logic is an outstanding solution for application of natural sequential, but it doesn't need a new operating system or language to implement it. On the other side, not all the functions are of sequential type. In regular PLCs it can enjoy both worlds as required and it'll explain in detail in the other article.
The hardware difference between PC based systems and PLC
The actual revolution in the last decade is the personal computer. If you remember the CPU of 286 based machines 10 years ago with 500Kb RAM, and you use a Pentium III machine with Mb RAM now you cannot believe it needs only for 10 years. Yet you need to remember that the PC is designed to run totally dependent on the customer. The design of BIOS always gets the user into concern. If something comes wrong, the user is then to reboot the system. Try to calculate the times you reboot your PC and you'll discover that it is over than once a week.
The PLC is designed to be built up in a cabinet of electrical unattended. Its working system executes a lot of self diagnostic tests every scan and the results of tests are accessible to the application program via "data registers" and "internal relays". The technique PLC handles power breaks is so much different than the method that PC does, that this topic alone imitate the theoretical difference between the both. The so-called "industrial PCs" are presently the same PC with the same theory behind the design of BIOS, in a better package of industrial.
Multi-Processors
The prospect as hear from the researchers of computer is in parallel processing. Parallel processing indicates multi-processors in one mechanism and a smart operating system that splits the jobs between the processors. PLC suppliers recommend the so-called "intelligent modules" for particular tasks like Communication, PID loops, Servo control, Stepper motor control, etc. every modules includes a divide CPU that runs separately parallel to the other modules and the main PLC CPU.