A Building Automation System (BAS) is an example of a Distributed Control System. Building automation describes the functionality provided by the control system. The building automation control system is a computerized, intelligent network of electronic devices, designed to monitor and control the mechanical and lighting systems in a building.

BAS core functionality keeps the building climate within a specified range, provides lighting based on an occupancy schedule, monitors system performance and device failures, and provides email and/or text notifications to building engineering staff. The BAS functionality reduces building energy and maintenance costs when compared to a non-controlled building. A building controlled by a Building Automation System is often referred to as an intelligent building system. The term most commonly used in the HVAC industry is DDC or Direct Digital Control.

A Direct Digital Control System (DDC) is often used to control HVAC (heating, ventilating, and air conditioning) devices such as valves via microprocessors using software to perform the control logic. Such building automation systems receive analog and digital inputs from the sensors and devices installed in the HVAC system and, according to the control logic, provide analog or digital outputs to control the HVAC system devices. These systems may be mated with a software package that graphically allows operators to monitor, control, alarm and diagnose building equipment remotely.

System Overview

Central controllers and most terminal unit controllers are programmable, meaning the direct digital control (DDC) program code can be customized for the intended use. The program features include time schedules, setpoints, controllers, logic, timers, trend logs, and alarms.

The unit controllers typically have analog and digital inputs that allow measurement of the variable (temperature, humidity, or pressure) and analog and digital outputs for control of the medium (hot/cold water and/or steam). Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or current measurement from a variable (temperature, humidity, velocity, or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or current signals to control the movement of the medium (air/water/steam) control devices.

Data Communication

When direct digital control (DDC) system controllers are networked together, they can share information through a data bus. The control system may speak ‘proprietary’ or ‘open protocol’ language to communicate on the data bus. Examples of open protocol language are BACnet (Building Automation Control Network), LON (Echelon), and Modbus.


When different direct digital control (DDC) system data networks are linked together, they can be controlled from a shared platform. This platform can then share information from one language to another. For example, a LON controller could share a temperature value with a BacNet controller. The integration platform can not only make information shareable, but can interact with all the devices.

Most of the integration platforms are either a PC or a network appliance. In many cases, the HMI (human machine interface) or SCADA (Supervisory Control And Data Acquisition) are part of it. Integration platform examples, to name only a few, are the Tridium Niagara AX, Johnson FX40,TAC Vista and the Unified Architecture i.e OPC (Open Connectivity) server technology used when direct connectivity is not possible.