Describe the function of a Direct Digital Control (DDC) controller in a BAS and how it is programmed.

A Direct Digital Control (DDC) controller in a Building Automation System (BAS) is a microprocessor-based device that controls the operation of HVAC equipment, lighting systems, and other building systems. The DDC controller receives input signals from sensors, processes the data according to a programmed control strategy, and sends output signals to actuators to control the equipment. Unlike older pneumatic or analog control systems, DDC controllers use digital signals and microprocessors to perform control functions, offering greater precision, flexibility, and programmability. The DDC controller's function is to maintain desired conditions, such as temperature, humidity, and lighting levels, by automatically adjusting the equipment in response to changing conditions. For example, a DDC controller might monitor the temperature in a room and adjust the position of a valve on a heating or cooling coil to maintain the desired temperature. DDC controllers are programmed using a graphical programming language or a text-based programming language. Graphical programming languages use drag-and-drop interfaces and pre-built function blocks to create control sequences. Text-based programming languages use code to define the control logic. The programming language allows the engineer to create a series of steps that the controller will follow to respond to sensor inputs. Common programming elements include proportional-integral-derivative (PID) loops, which are used to precisely control process variables; logic gates, which are used to implement Boolean logic; and timers and schedules, which are used to control equipment based on time of day or day of week. The programmed control strategy is downloaded to the DDC controller, where it is executed continuously.