The Ddc Dictionary

ALGORITHM—An assortment of rules or steps presented for solving a problem.

ANALOG INPUT—A variable input that is sensed, like temperature, humidity or pressure and is sent to a computer for processing.

ANALOG OUTPUT—A variable signal such as voltage, current pneumatic air pressure which would in turn operate a modulating motor which will drive valves, dampers, etc.

ANALOG TO DIGITAL A/D CONVERTER—An integrated circuit that takes data such as a temperature sensor and converts it into digital logic that the microprocessor can recognize. All analog inputs of a DDC system are fed into the microprocessor through an A/D converter.

BAUD RATE—The speed, in bits per second, at which information travels over a communications channel.

BIT—The smallest representation of digital data there is. It has two states; 1 (on) or 0 (off).

BITS OF RESOLUTION—A representation of how finely information can be represented. Eight bits of resolution means information may be divided into 256 different states, ten bits allows division into 1024 states and twelve bits allows division into 4096 states.

BYTE—Eight bits of digital information. Eight bits can represent up to 256 different states of information.

CONTROL LOOP—The strategy that is used to make a piece of equipment operate properly. The l oop receives the appropriate inputs and sets the desired condition.

DAISY CHAIN—A wiring scheme where units must follow one another in a specific order.

DEAD BAND—An area around a setpoint where there is no change of state.

DEAD TIME—A delay deliberately placed between two related actions in order to avoid an overlap in operation that could cause equipment problems.

DERIVATIVE CONTROL—A system which changes the output of a controller based on how fast a variable is moving from or to a setpoint.

DIGITAL INPUT—An input where there are only two possibilities, such as on/ off, open/ closed.

DIGITAL COMMUNICATION BUS—A set of wired, usually a twisted pair for DDC controllers. Information is sent over this bus using a digital value to represent the value of the information.

DIGITAL OUTPUT—An output that has two states, such as on/off, open/closed.

DIGITAL TO ANALOG D/A CONVERTER—An integrated circuit that takes data from the microprocessor and converts it to analog data that is represented by a voltage or current. All analog outputs of a DDC system go through a D/A converter.

DISTRIBUTED CONTROL—A system where all intelligence is not in a single controller. If something fails, other controllers will take over to control a unit.

EPROM—Erasable Programmable Read Only Memory. An integrated circuit that is known as firmware, where software instructions have been "burned" into it.

EEPROM—Electrically Erasable Programmable Read Only Memory. An integrated circuit like the EPROM above except that the information may be altered electronically with the chip installed in a circuit.

FCU—Fan Coil Unit. A type of terminal unit.

FEEDBACK—The signal or signals which are sent back from a controlled process telling the current status.

FIRMWARE—Software that has been programmed into an EPROM.

FLOATING POINT—Numerical data that is displayed or manipulated with automatic positioning of the decimal point.

HP—Heat Pump. A type of terminal unit.

INPUT—Data which is supplied to a computer or control system for processing.

INTEGRAL CONTROL—A system which changes the output of a controller based on how long a variable has been offset from a setpoint. This type of control reduces the setpoint/variable offset.

INTELLIGENT BUILDING—A building that is controlled by a DDC system.

LAG—The delay in response of a system to a controlled change.

LAN—Local Area Network. A communication line through which a computer can transmit and receive information from other computers.

MICROPROCESSOR—The brains of all DDC systems and, for that matter, personal computers. An integrated circuit which has logic and math functions built into it. If fed the proper instructions, it will perform the defined functions.

MODEM—A device which allows the computer to communicate over phone lines. It allows the DDC system to be viewed, operated and programmed through a telephone system.

OFFSET—The difference between a variable and its setpoint.

OPERATOR'S TERMINAL—The computer at which the operator can control a motor or to a relay to start/stop a unit.

OUTPUT—Processed information that is sent by a controller to an actuator to control a motor or to a relay to start/stop a unit.

PI CONTROL—A combination of proportional and integral control. Adequate for almost all HVAC control applications. Control loops using PI control to look at both how far an input is from setpoint and how long it has been away from setpoint.

PID CONTROL—Proportional plus Integral plus Derivative Control. A system which directs control loops to look at how far away an input is from setpoint, how long it has been at setpoint and how fast it is approaching/moving away from setpoint.

PROPORTIONAL CONTROL—A system which linearly varies the output as an input variable changes relative to setpoint. The farther the input is from setpoint, the larger the change in the output.

PULSE WIDTH MODULATION—A means of proportionally modulating an actuator using digital outputs. One output opens the motor, another closes it. Used extensively in VAV boxes for driving the damper motor on the box.

RAM—Random Access Memory. A computer chip that information can be written to and read from. Used to store information needed in control loop calculations.

SETPOINT—A value that has been assigned to a controlled variable. An example would be a cooling setpoint of 76 degrees F.

SOFTWARE—The list of instructions written by an engineer or programmer that makes a controller or computer operate as it does.

TERMINAL UNIT—Part of the mechanical system that serves an individual zone, such as VAV box, hydronic heat pump, fan coil, etc.

TRANSDUCER—A unit that converts one type of signal to another. An electronic signal to a pneumatic signal.

TWISTED PAIR—Two wires in one cable t hat are twisted the entire length of the cable. DDC systems often use a twisted pair for communication link between controllers.

TWO POSITION—Something that has only two states. A two position valve has two states, either open or closed.

USER INTERFACE—The operator's terminal is usually referred to as the user interface. I f allows the operator to interrogate the system and perform desired functions. Most interfaces today are personal computers or minicomputers.

VAV—Variable Air Volume. A terminal unit that varies the amount of air delivered to a space, depending upon the demand for cooling.

WORD—Sixteen bits of digital information. Sixteen bits can represent up to 65,536 states of information.

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