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In view of this, a second LAN and file server are crucial to help ensure system stability, even in the event of a network failure. If the LAN in the newly configured system also fails, then control and monitoring by the display nodes is lost. Having secured the system by removing the single point of failure (the I/O server) the user could be excused for thinking that all system eventualities had been covered. In addition, it is possible to support dual network paths to the centralised database, allowing dual file servers if required. Changes only need to be made to one database and are then automatically updated everywhere else. Another benefit is that centralised databases are easier to manage and maintain. If a dedicated file server is also added to the SCADA system, the user can centralise the databases and display screens continuity is then maintained if the primary server fails. Data is automatically backfilled and the two servers become synchronised again as the standby server reverts to its former role. When the primary server is repaired and returned to service, it reads the plant's status from the standby server and resumes its role as the primary server. If communication is interrupted, the standby server assumes the primary server has failed and takes over the role as the primary server. The primary server also communicates with the standby server, continually updating the plant's status. A better solution for a client/server system is one in which only the primary server communicates with the PLCs. In that scenario, both servers would have to communicate with the PLCs, thus doubling the load on the PLC network and reducing system performance. The standby server does not duplicate the primary server's functions. In very large installations, host pairs of servers are used with one host pair dedicated as a standby in a separate location from the primary host pair. If the primary server fails, the client's requests for data are channelled to the standby server. To provide redundancy, a second standby server can be added that is also dedicated to communication with plant control devices.
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When a client computer requires data for display, it requests data from the server and processes that data locally. In a simple application, the computer connected to the control and monitoring units becomes the server that is dedicated to communication with the plant control devices, while the display nodes are clients. The benefit of this is that it increases the speed and efficiency of the system by distributing the processes in the control and monitoring application across two or more computers (using a LAN).
HOW TO TEST SCREEN WITH CITECT WINDOWS
In 1992, Citect for Windows started using a client/server architecture for plant monitoring and control. SCADA specialist, Citect offered systems with built-in redundancy nearly 15 years ago, and its first redundant installation is still in operation today. However, whatever type of disaster recovery is planned for, it is possible to avoid lost data and reduce downtime by planning the proper system design, and by choosing a SCADA system with built-in redundancy.
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In the first instance it is important to determine what level of redundancy is required: hardware failure, catastrophic failure, energy failure or a natural disaster? Mission critical installations often have separate power sources in case of a power failure, and installations in areas prone to natural disasters or the threat of fire separate the servers in different geographic locations. Consequently, if some or all of the plant processes are critical, or if downtime costs are high, redundancy must be incorporated into the system to avoid the consequences of equipment failure. Granted, most modern computers are designed for reliability, but breakdowns still occur, especially with computers located in harsh environments. The question is particularly relevant when one considers that control systems for both single node and network applications have a single point of failure, meaning that they will break down entirely if one piece of hardware fails (such as when the computer is connected to the control and monitoring units). Put more simply: what happens if the system fails? Generally SCADA systems are highly reliable however, one aspect in their operation that is often overlooked at the specification stage is redundancy. SCADA systems are widely used in plants and factories across the world the advanced high level control and monitoring features they provide being fundamental to improving plant efficiency and productivity. Steve Flannagan explains how production systems can be protected against a remote but possible eventuality - the failure of a supervisory control and data acquisition (SCADA) system