Direct Digital Control

Heating, ventilation, and air conditioning has usually been controlled by three basic systems: pneumatic, electric, electronic, but another technology, known as direct digital control (DDC), can lower costs and improve comfort in a vast range of complex applications. DDC is an electronic system that uses a digital microprocessor (computer) to analyze various parameters in HVAC, such as temperatures, pressures, air flows, time of day, and many other types of data. The information obtained is used to operate control devices, such as dampers and valves, and to start / stop mechanical equipment such as air-handling units and heating and cooling pumps, in accordance with instructions programmed into the microprocessors memory.
DDC also offers features in addition to the basic functions, such as monitoring and alarm, data gathering, and energy monitoring and management. DDC systems also accommodate lighting-control and security-system functions and management.
They can delegate all of the control logic, including determination of necessary output signals, to be transmitted to the control devices and to the microprocessor.
Development of lower-cost chips (computers on a chip) may reduce limitations on the number of devices a single microprocessor can handle.
DDC technology tends toward distributed control, which permits multiple controllers to be used, each with its own microprocessor. This allows each controller to operate independently; after it has been programmed, it can operate independently.
A twisted-wire pair is usually used as a data link for connecting the controllers together. This permits exchange of information with each other or with a host computer and, in many cases, with desktop personal computers (PCs).
The host computer is connected to the distributed controllers and is considered as the operators link. It permits the user to exchange information on the status of any portion of the HVAC system and to revise control schedules, values, or setpoints.
When the host computer is not being used for these tasks, it can be used with PCs for usual or conventional tasks, while the controllers continue to operate with their most current set of instructions.
Capacity of a controller is usually measured by its point capacity. Input or output points may be either analog or digital classifications. Analog inputs are those with values that change with time, such as temperatures and pressure. Digital outputs are those that describe a condition that may be of two types only such as on and off.
The advantages of DDC compared with pneumatic control systems are numerous.
The greatest advantage is the ability to achieve impressive energy conservation without sacrificing comfort. DDC permits a system to be controlled in an optimum manner while maintaining space temperatures within desired limits. Also, DDC is an extremely flexible control system that permits an operator to control the parameters and control sequence to maintain maximum efficiency. On large installations with many buildings, air-handling units may be reprogrammed for readjustment of parameters for improved comfort conditions or optimal energy conservation, or both. These requirements are simpler to meet with a DDC system than with a pneumatic system.
An analog output signal is used for modulating dampers or varying flow quantities through a liquid-control valve. To start and stop electrical equipment, a digital output is used and is usually a dry type of contact.
As an alternative, a mix of DDC and pneumatic actuators may be used on the controlled devices, but electronically operated. In some cases, use of electronic  actuation may be economical, particularly for larger valves and dampers. When a pneumatic output signal is required from the controller, a simple digital-topneumatic relay converts the electronic signal to a variable pressure signal. Pneumatic actuators provide the additional advantages of excellent reliability and low maintenance cost.

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