DDC Control System Definitions:

1. Algorithm: A logical procedure for solving a recurrent mathematical problem. A prescribed set of well-defined rules or processes for the solution of a problem in a finite number of steps.

2. Analog: A continuously varying signal value (e.g. temperature, current, velocity)

3. Baud: A Baud is a signal change in a communication link. One signal change can represent on or more bits of information depending on type of transmission scheme. Simple peripheral communication is normally one bit per Baud. (e.g., Baud Rate = 9600 Baud/sec is 9600 bits/sec if one signal change = 1 bit.

4. Binary: A two-state system where an "on" condition is represented by a high signal level and an "off" condition is represented by a low signal level.

5. Control Wiring: Includes conduit, wire and wiring devices to install interlocks, thermostats, PE and EP switches, and like devices. Includes all wiring from a DDC cabinet to all sensors and points defined in the input/output summary shown on drawings and required to execute the sequence of operation.

6. Digital Controller (CU): A control module which is microprocessor based, programmable by the user, may include I/O data processing functions. There could be more than one type of CU for specific applications. For example, Auxiliary Units (ACU’s) for air handling units, unitary control units (UCU’s) for VAV boxes, fan coil units, etc., and Remote Control Units (RCU’s) for supervising ACU’s and UCU’s, etc.

7. Distributed Control Panel (DCP): A panel which houses the digital controller (CU), input and output functions, power supplies, relays, transducers, and associated hardware.

8. Direct Digital Control (DDC): A control loop or subsystem in which digital and analog information is received and processed by a microprocessor based system, and digital control signals are generated based on control algorithms and transmitted to field devices in order to achieve a set of predefined conditions.

9. Deadband: A temperature range over which no heating or cooling is supplied i.e. 22-25 ° C (72-78 ° F), as opposed to a single point changeover or overlap).

10. Diagnostic Program: A software test program which is used to detect and report system or peripheral malfunctions and failures. This test is performed at the initial start up of the system.

11. Distributed Control System: A system in which the processing of system data is decentralized and control decisions can and are made at the subsystem level. System operational programs and information are provided to the remote subsystems and status is reported back to the Engineering Control Center (ECC). Upon the loss of communication with the EEC, the subsystems shall be capable of operating in a standalone mode using the last best available data. Digital controllers in a system are linked in a communications network composed of one or more levels of area networks (LAN).

12. Down Load: The electronic transfer of programs and data files from a central computer or operator workstation with secondary memory devices, to remote, distributed computers in a distributed system. Transfer is made over the distributed computers in a distributed system. Transfer is made over the distributed system’s communication network.

13. Dynamic Control: A process that optimizes operation of HVAC systems (air handling units, converters, chillers, etc.) by increasing and decreasing setpoints or starting and stopping equipment in response to heating and cooling loads of downstream equipment. A requirement of dynamic control is knowing the heating/cooling demand status of downstream equipment, therefore dynamic control requires controllers connected in a communications network.

14. Engineering Control Center (ECC): The centralized control point for the intelligent control network. The ECC includes operator Workstation(s), network communications control. Operator Workstation includes central Operator’s personal-computer based terminal, keyboard, mouse, printer(s), and any additional peripheral devices required to perform the primary man-machine functions of the ECC.

15. Firmware: Firmware is software programmed into read only memory (ROM) chips. Software may not be changed without physically altering the chip.

16. Graphic Sequence of Operation: A drawing or graphic showing all interlocks and control loop sequences between the input and output points. Graphic sequence of operation is a graphical representation of the sequence of operation. The graphic sequence of operation will show all inputs, outputs, and logic blocks.

17. Input/Output (I/O): I/O refers to analog inputs (AI), digital inputs (DI), analog output (AO), and digital output (DO) in a digital controller. Inputs from analog sensors (temperature, pressure, flow, humidity, etc.) and digital sensors (motor status, flow switches, switch position, and pulse output devices).

18. Input/Output Unit: The section of a DDC system through which information is received and transmitted.

19. Man-machine Interface: A method by which an operator is capable of communicating with a DDC system. In the case of a computer, the man-machine interface includes: the keyboard, mouse, monitor, and so on. The Man-machine interfacing allows an operator to command, control, monitor, and program the system.

20. Local Area Network (LAN): A communications bus that interconnects digital controllers for peer-to-peer communications. A LAN shall allow sharing of global information, make it possible to apply building wide control strategies, such as peak demand limiting, permit dynamic control strategies, allow coordinated response to alarm conditions, and permit remote monitoring and programming of digital controllers.

21. Operating System (OS): Software which controls the execution of computer application programs.

22. Peripheral: Input/Output unit used to communicate with the computer, digital controllers, and make copies of system outputs. Peripherals include CRT, printer, tape deck, and diskette.

23. Microprocessor: A microprocessor refers to the central processing until (CPU) that contains all the registers and logic circuitry that make it possible for digital controllers to do computing.

24. Peer-to-Peer: Peer-to-Peer refers to controllers connected on a communications LAN that act independently, as equals, and communicate with each other and pass information which facilitates control.

25. Resolution: Resolution refers to the number of possible states that input value or output can take and is functional of the digital controller I/O circuitry; the A/D converter for input and the D/A converter for output. Sixteen (16) bit resolution has 65536 possible states and eight (8) bit resolution has 256 possible states.

26. Stand-Alone Control: Stand-Alone Control refers to the digital controller being able to perform required climate control and energy management functions without connection to another digital controller or central site computer. Digital controller requirements for stand-alone control are a time clock, a microprocessor, microchip resident control programs, PID control, a communications port for interfacing with and programming the control, firmware for interrogation and programming, and I/O for sensing and effecting control of its control environment.

27. Virtual Point: A virtual point allows transfer of calculated values between software programs, such as duty cycling, enthalpy, etc. This point resides only in software.

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