Scada should be spelt with two fs for fast and flexible

March 2000 SCADA/HMI

Getting information quickly and reliably to and from the plant floor is the primary role of any scada and HMI system. A system where data transfer is slow because of bottlenecks runs the risk of losing information. Citect, however, automatically uses dynamics optimisation to maximise system performance.

Each I/O device uses a unique protocol to communicate with higher-level equipment such as Citect. The speed with which data can be transferred, depends on, and is limited by, the I/O device and the protocol design. The limitation comes from the fact that I/O devices do not respond immediately to request for data, and many different protocols to requests for data, making the process inefficient. The following strategies allow Citect to maximise data transfer.

Citect's Communication is demand based - reading only those points that are requested by the clients. More importantly, the I/O server rationalises requests from clients, for example, combining them into one request where possible. This reduces the needless communication, giving screen update times up to eight times faster (than without). Only a restricted volume of data can be returned in one request. If all requested data are grouped together, then fewer requests are required, and the faster the response. But what happens when two required registers are separated? Citect uses a blocking constant to calculate whether I is quicker to read the separately, or in the same 'block'. By compiling a list of registers that must be read in one scan, Citect automatically calculates the most efficient way of reading the data.

The client server processing of Citect allows further performance increases, through the use of a cache on I/O server. When an I/O server reads registers, their values are retained in its memory for a user-defined period (typically 300 ms). If a client requests data that is stored in the cache, the data is provided without the register being re-read. In a typical two-client system, this will occur 30% of the time. The potential performance increase is therefore 30%. Citect also uses read ahead caching, updating the cache if it gets accessed - predicting that the same information will be requested again.

Scalable architecture: Scalability is the power to resize the system - up or down –without having to modify any of the existing system hardware or software. Citect's innovative scalable architecture allows the systems architecture to grow with the user's requirements, while preserving the initial investment.

Citect makes the most of its task oriented client-server design, allowing users to allocate tasks as they add more Citect computers. For example, if a second HMI is required, this is implemented by adding a LAN and a new computer, and nominating it as a display client. The new computer can share the same configuration, and will receive I/O from the first Citect.

A secondary benefit of doing this is that processing load may also be distributed. For example, if it is found that the first Citect computer is too busy, a second can be nominated to take care of the alarms by becoming the alarms server.

Put simply, some users want simple HMI, while others want a complete scada system. With Citect, users can do both; start small and later get the big picture.

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