The data handling, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking. The CPU runs memory routines, constantly checking the PLC (PLC controller is redundant) to avoid programming errors and ensure the memory is undamaged.
The CPU directs the PLC to execute control instructions, communicate with other devices, carry out logic and arithmetic operations, and perform internal diagnostics. It is a -16 or -32 bit microprocessor consisting of a memory chip and integrated circuits for control logic, monitoring, and communicating. PLC Hardware Components A central processing unit (CPU) serves as the brain of the PLC. In some models of programmable controller, the program is transferred from a personal computer to the PLC through a programming board which writes the program into a removable chip such as an EPROM.
IN A MODERN PLC, THE LADDER LOGIC PROGRAM (USER PROGRAM) IS USUALLY STORED IN SOFTWARE
The software will upload and download the PLC program, for backup and restoration purposes. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation. In some software packages, it is also possible to view and edit the program in function block diagrams, sequence flow charts and structured text. The programming software allows entry and editing of the ladder-style logic. The computer is connected to the PLC through USB, Ethernet, RS-232, RS-485, or RS-422 cabling. More recently, PLCs are programmed using application software on personal computers, which now represent the logic in graphic form instead of character symbols. The oldest PLCs used non-volatile magnetic core memory. Facilities for printing and documentation were minimal due to lack of memory capacity. Programs were stored on cassette tape cartridges. Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Early PLCs, up to the mid-1990s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs.