Engineering Manual o.. - HVAC.Amickracing

BUILDING MANAGEMENT SYSTEM FUNDAMENTALSSYSTEM-LEVEL CONTROLLERSMicroprocessor-based system-level controllers have greatercapacity than zone-level controllers in terms of number ofpoints, DDC loops, and control programs. System-levelcontrollers are usually applied to major pieces of mechanicalequipment such as large built-up air handlers, central VAVsystems, and central chiller plants. These controllers can alsoperform lighting control functions. Controllers at this levelinterface with controlled equipment directly through sensorsand actuators or indirectly through communications links withzone-level controllers. System-level controllers typically havea port for connecting portable operating and programmingterminals during initial setup and subsequent adjustments.When system-level controllers are linked to operations-levelprocessors, subsequent changes to controller programs arenormally made at the operations-level processor and then downlineloaded to the controller using the system transmission lines.System-level controllers also provide system survivability byoperating in a stand-alone mode should its communicationlink be lost.Some types of system-level controllers also provide theproperty and life-safety protection for the facility through firealarm panels, security panels, and access control panels. SeeINTEGRATION OF OTHER SYSTEMS.OPERATIONS-LEVEL PROCESSORSOperations-level processors interface primarily with BMCSoperating personnel. The processor at this level is in most casesa PC with color operator terminal displays and plug-in functionboards to accommodate additional operator terminals, printers,memory expansion, and communications links. An operationslevelprocessor generally includes application software for:— System security: Limits access and operation toauthorized personnel.— System penetration: Permits authorized personnel toselect and retrieve system data via PC keyboard or otherselection mechanism.— Data formatting: Assembles random system points intological group format for display and printout.— Data segregation: Groups points by major point typesfor routing to a specific terminal and specified operator.— Custom programming: Develops custom DDC programsat the operations level for down-line loading to specific,remote system-level controllers and zone-level controllers.For more information on custom programming see theMicroprocessor-Based/DDC Fundamentals section.— Graphics: Builds custom graphic displays incorporatingdynamic system data. Bar chart and curve plot softwaremay be included.— Standard reports: Provides automatic, scheduled, and byrequestreports of alarm and operator activity. Alsoprovides a broad range of system and category (pointsin-alarm,disabled points, etc.) summary reports.— Custom reports: Provides spread sheet, word processing,and a data base management capability.— Maintenance management: Automatically schedules andgenerates work orders for equipment maintenance basedeither on history of equipment run time or on a calendarschedule.— Site-specific customization: Allows defining operatorassignments, peripheral device assignments, printer datasegregation, system configuration, display and printouttext of action message assignments to specific points,time/holiday scheduling, point monitoring/control, time/event program assignments, and application programparameter assignments.— System integration: Provides common control andinterface for multiple subsystems (HVAC, fire, security,access control) and provides global activity as a result ofspecific subsystem events (e.g., closing or openingdampers to control smoke as a result of a fire alarm).MANAGEMENT-LEVEL PROCESSORSManagement-level processors, at the top of the BMCS systemhierarchy, exercise control and management over the connectedsubsystems. An operator at this level can request data from andissue commands to points anywhere in the system (as with mostoperations-level processors). Day-to-day operation is normally afunction of the operations-level processor; however, completecontrol can be transferred to the management-level processorduring emergencies or unattended periods. The management-levelprocessor primarily collects, stores, and processes historical datasuch as energy use, operating costs, and alarm activity, andgenerates reports that provide a tool for the long-term managementand use of the facility.COMMUNICATIONS PROTOCOLCommunications protocol is an essential element of theBMCS configuration due to the amount of data transferredfrom one point to another and because distributed processorsmay be dependent on each other for data pertinent to residentprograms. Communications links, or buses, generally use eithera poll/response or a peer protocol. Early BMCSs use pollresponseprotocols where most system intelligence and dataprocessing was at the central processor. In the mid 1990s mostBMCSs use peer protocols which share the communicationsbus equally among all bus devices with no master device.ENGINEERING MANUAL OF AUTOMATIC CONTROL 187