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SYSTEM-LEVEL CONTROLLERS
Microprocessor-based system-level controllers have greater
capacity than zone-level controllers in terms of number of
points, DDC loops, and control programs. System-level
controllers are usually applied to major pieces of mechanical
equipment such as large built-up air handlers, central VAV
systems, and central chiller plants. These controllers can also
perform lighting control functions. Controllers at this level
interface with controlled equipment directly through sensors
and actuators or indirectly through communications links with
zone-level controllers. System-level controllers typically have
a port for connecting portable operating and programming
terminals during initial setup and subsequent adjustments.
When system-level controllers are linked to operations-level
processors, subsequent changes to controller programs are
normally made at the operations-level processor and then down-
line loaded to the controller using the system transmission lines.
System-level controllers also provide system survivability by
operating in a stand-alone mode should its communication
link be lost.
Some types of system-level controllers also provide the
property and life-safety protection for the facility through fire
alarm panels, security panels, and access control panels. See
INTEGRATION OF OTHER SYSTEMS.
OPERATIONS-LEVEL PROCESSORS
Operations-level processors interface primarily with BMCS
operating personnel. The processor at this level is in most cases
a PC with color operator terminal displays and plug-in function
boards to accommodate additional operator terminals, printers,
memory expansion, and communications links. An operations-
level processor generally includes application software for:
— System security: Limits access and operation to
authorized personnel.
— System penetration: Permits authorized personnel to
select and retrieve system data via PC keyboard or other
selection mechanism.
— Data formatting: Assembles random system points into
logical group format for display and printout.
— Data segregation: Groups points by major point types
for routing to a specific terminal and specified operator.
— Custom programming: Develops custom DDC programs
at the operations level for down-line loading to specific,
remote system-level controllers and zone-level controllers.
For more information on custom programming see the
Microprocessor-Based/DDC Fundamentals section.
— Graphics: Builds custom graphic displays incorporating
dynamic system data. Bar chart and curve plot software
may be included.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING MANAGEMENT SYSTEM FUNDAMENTALS
— Standard reports: Provides automatic, scheduled, and by-
request reports of alarm and operator activity. Also
provides a broad range of system and category (points-
in-alarm, disabled points, etc.) summary reports.
— Custom reports: Provides spread sheet, word processing,
and a data base management capability.
— Maintenance management: Automatically schedules and
generates work orders for equipment maintenance based
either on history of equipment run time or on a calendar
schedule.
— Site-specific customization: Allows defining operator
assignments, peripheral device assignments, printer data
segregation, system configuration, display and printout
text of action message assignments to specific points,
time/holiday scheduling, point monitoring/control, time/
event program assignments, and application program
parameter assignments.
— System integration: Provides common control and
interface for multiple subsystems (HVAC, fire, security,
access control) and provides global activity as a result of
specific subsystem events (e.g., closing or opening
dampers to control smoke as a result of a fire alarm).
MANAGEMENT-LEVEL PROCESSORS
Management-level processors, at the top of the BMCS system
hierarchy, exercise control and management over the connected
subsystems. An operator at this level can request data from and
issue commands to points anywhere in the system (as with most
operations-level processors). Day-to-day operation is normally a
function of the operations-level processor; however, complete
control can be transferred to the management-level processor
during emergencies or unattended periods. The management-level
processor primarily collects, stores, and processes historical data
such as energy use, operating costs, and alarm activity, and
generates reports that provide a tool for the long-term management
and use of the facility.
COMMUNICATIONS PROTOCOL
Communications protocol is an essential element of the
BMCS configuration due to the amount of data transferred
from one point to another and because distributed processors
may be dependent on each other for data pertinent to resident
programs. Communications links, or buses, generally use either
a poll/response or a peer protocol. Early BMCSs use poll-
response protocols where most system intelligence and data
processing was at the central processor. In the mid 1990s most
BMCSs use peer protocols which share the communications
bus equally among all bus devices with no master device.
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