changing signal from an external device or sensor, such as a temperature sen-
sor. Digital inputs to a controller are simply a two-state, on-off signal from
external devices or sensors, such as a switch.
In much the same way, analog outputs from a controller are “proportional
variable” signals sent by the controller to adjust an actuator or external con-
trol device, such as a valve actuator. Digital output from a controller is a
two-state or two-position signal from the controller to an actuator, such as a
control fan relay start-stop switch.
Most field devices and equipment for building automation systems commu-
nicate at low network speeds, typically transmitting at rates of less than 1
Mbps. The communications network for a building automation system is typ-
ically in a physical star or bus topology from the controller.
Older HVAC system controls were provided through electric power or
pneumatic means. However, direct digital control (DDC) is commonly used
in more complex HVAC systems. DDC allows for a system controller to com-
pute the sequence of operations based on the digital input from system sensors.
Although DDCs are digital controls, they are able to handle analog-to-digital
and digital-to-analog conversions. Unlike electric or pneumatic controls, DDC
can be programmed for any sequence of operations.
Controllers can, confusingly, be referred to in a variety of ways: master,
slave, terminal, floor, and others. HVAC system network architecture typically
consists of the following network levels:
n Management level
n System-level or building-level controllers
n Field-level controllersManagement Level
The top level of an HVAC control system is the management level consisting
of personal computers or multiple PCs connected via an Ethernet network.
These operator workstations can communicate with, interrogate and control
any of the controllers and devices on the network. The management level pro-
vides many functions:
n Administration and control of the HVAC system
n Programming for the system and other controllers, including operation
sequences
n Display of system information44 Smart Building Systems for Architects, Owners, and Builders