The systems of PLC offer some “challenges” for engineers who maintain and design control systems. The PLC and every PC must be constructed and programmed independently, and then connected together to make the complete control system. This is recognized as the “multiple database trap”, one of the major sources of human error in maintaining and designing a PLC system is multiple places that the similar data have to be entered to construct a system. In a system of PLC, each PLC and PC component must be configured and programmed individually:
• The devices of input and output have to be mapped into the data table of PLC by bit position, file, and word location.
• The user can allocate tag names to every I/O point to program the system in most PLC programming software packages.
• The Human-Machine Interface (HMI) must be constructed to map the locations of I/O inside the PLC to the screen of operator. If tag names are employed in the HMI device, they must be entered a second time into the database of HMI.
• If a system of supervisory control and data acquisition is used to log & collect data, then a driver must be constructed to set up register transfers from the PLC to the system of data acquisition.
The data must be entered into each of the four different databases, each database must be adapted and tested against the other databases, without error to acquire the change to function properly if any changes are made. The system of PLC leaves a lot to be wanted when it approaches to eliminating human error and lowering data entry.
The PC-based Control Systems Advantages
The system design and maintenance has turn into radically simpler with the advent of PC-based controls. The PLC functions, operator interface, programming terminal, and data acquisition can be merged into a single PC with a Windows-based PC control system.
The three PCs and the PLC needed to control the system have been incorporated into a single PC with PC-based control. The power is accessible to incorporate the functions of a PLC, operator interface, programming, and data acquisition onto a single platform with the advent of the Windows and higher-speed PCs. The clear cost savings from reducing an expensive proprietary PLC and several PCs, there are a number of important technical advantages when shifting to a PC-based Control system:
• commodity I/O mix & match,
• Leveraging the platforms of mainstream computing
• Highly developed, easier to operate Windows-based programming tools
• Reducing the multiple database trap