2245Digital Control also called Direct Digital Control or DDC

This control technology uses microprocessors to provide control. A big advantage to DDC controls is the fact that changes to the system are often made with software and do not automatically require physical changes and cost like other technologies. Discrete (on-off) information is readily absorbed as a "1" or "0." For analog processing, interface equipment called digital-to-analog converters (D/A) and analog-to-digital converters (A/D) are used. The higher the resolution of the A/D and D/ A conversion, the closer the digital signals resemble true analog signals, allowing smoother control.

One of the great enhancements of digital controls over the last 30 years has been the concept of distributed control. This technology shift occurred in response to customer complaints of excessive dependence on single hardware or software points, and widespread loss of control after a single item failure. Current best practice uses multiple smaller controllers at the points of control. These communicate upstream to a supervisory operator workstation, but each single failure point now affects a much smaller area, increasing overall process reliability.

• DDC Graphics: A Graphical User Interface (GUI) is an overlay onto most DDC control systems. While labor-intensive to create, these are useful for gaining acceptance of the control system, and can reduce the skill level needed for digital control system navigation. The GUI uses easily recognized icons and symbols, color-coded messages and alarms, and other visual methods, to increase the user-friendliness of the system. The GUI is analogous to the labeled display boards of traditional pneumatics.

Field User Interface: For effective operations and maintenance, the user interface should not be limited to operator workstation graphics. Like the pressure gages or test ports of a pneumatic system, some form of user interface should be provided at the point of control to invite personnel to interact. This may be a fixed display-adjusting device that can scroll through a few points, to a portable operator workstation that can be connected at will. Without some provision for user interface in the field, the DDC system can introduce a frustration to the operations personnel who cannot "see" what is going on at the equipment locations.

DDC Controller Hardware: Today's digital controls come in a variety of styles, to suit the application. For common applications, mass produced and low cost definite purpose controllers are available. These use factory programming, also called 'canned' software, which speeds application and start-up, but may limit the opportunities to create special instructions or for optimization, unless sufficient programming is available from the supervisory controller to achieve the intended result. Multi-purpose generic controllers are also available that utilize custom programming. These generally come with a fixed number of input and output points, but the point capacity can often be expanded with multiplexers or other point expander cards. Since the programming is custom, these controllers are the natural choice for optimization routines that require dynamic calculations and adaptability. For control processes with a large number of points, point expander boards are usually preferable to multiple linked controllers because having a single processor brain eliminates some unpredictable operations modes that come from partial failures, e.g. if just one of the controllers fails. Conversely, too many expander boards can turn this application into a large-effect single point of failure.

Proprietary vs. Open DDC Controls: The topic of proprietary controls, captive customers, open protocols, hybrid systems, gateways, etc. is a hot topic in the control industry today. While the concept is not difficult to grasp, details and implications are very numerous and complex, and beyond the scope of this text. The basic argument FOR proprietary systems is the one-stop-shopping ease of purchasing, and security of all-encompassing technical resources for system maintenance and system integration; for large or complex systems this can be an important benefit. The basic argument AGAINST proprietary systems is the proprietary lock on the customer, and how the lack of competition almost certainly results higher replacement costs that escalate during the ownership life cycle.

Current state of the industry includes both proprietary systems and "open protocol" systems, and both are viable, requiring a customer choice where new systems are proposed. Gateways and translator offerings are hardware go-between solutions that offer limited connectivity between proprietary systems. These are common solutions, but can create new problems even as they solve others—and many retain the proprietary nature they are intended to solve.

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