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whats diffeence between dcs & scada operation.
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Answer Posted / likhit
The goals of DCS and SCADA are quite different. It is possible for a single system to be capable of performing both DCS and SCADA functions, but few have been designed with this in mind, and therefore they usually fall short somewhere. It has become common for DCS vendors to think they can do SCADA because the system specifications seem so similar, but a few requirements paragraphs about data availability and update processing separates a viable SCADA system from one that would work OK if it weren't for the real world getting in the way.
DCS is process oriented: it looks at the controlled process (the chemical plant or whatever) as the centre of the universe, and it presents data to operators as part of its job. SCADA is data-gathering oriented: the control centre and operators are the centre of its universe. The remote equipment is merely there to collect the data--though it may also do some very complex process control!
A DCS operator station is normally intimately connected with its I/O (through local wiring, FieldBus, networks, etc.). When the DCS operator wants to see information he usually makes a request directly to the field I/O and gets a response. Field events can directly interrupt the system and advise the operator.
SCADA must operate reasonably when field communications have failed. The 'quality' of the data shown to the operator is an important facet of SCADA system operation. SCADA systems often provide special 'event' processing mechanisms to handle conditions that occur between data acquisition periods.
There are many other differences, but they tend to involve a lot of detail. The underlying points are:
SCADA needs to get secure data and control over a potentially slow, unreliable communications medium, and needs to maintain a database of 'last known good values' for prompt operator display. It frequently needs to do event processing and data quality validation. Redundancy is usually handled in a distributed manner.
DCS is always connected to its data source, so it does not need to maintain a database of 'current values'. Redundancy is usually handled by parallel equipment, not by diffusion of information around a distributed database.
These underlying differences prompt a series of design decisions that require a great deal more complexity in a SCADA system database and data-gathering system than is usually found in DCS. DCS systems typically have correspondingly more complexity in their process-control functionality.
The company I work for has both DCS and SCADA products. The operator stations for each product line can use the same UNIX workstations. The systems share data (and thus form a composite SCADA/DCS system), but the SCADA database architecture is significantly different from the DCS data architecture, to the extent that the SCADA master station database looks to the DCS operators very much like some directly-connected DCS I/O. The DCS people are (of course) keen to simplify this to cut costs. However, they do not yet have a viable alternative for the mechanisms required in SCADA systems to have communications redundancy and data redundancy to provide the sort of SCADA system reliability that our customers expect.
If you look at most customer's system requirements specifications, a careful analysis of the data collection and data quality requirements will indicate if SCADA-style or DCS-style systems are appropriate. In general: the more features a system provides the more it will cost, so if you do not need SCADA-type data gathering facilities it will usually be more economical to use a DCS-type system. If you do need these facilities, you will pay for them.
The short answer: DCS and SCADA are still different things, it depends what the customer specifies as to which is appropriate for a particular installation.
I hope this has clarified more than it has confused. Also, it is my opinion based on my own experiences with DCS and SCADA. Others may have experience with systems that are designed to provide full SCADA and full DCS functionality in the one system.
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