"INTRODUCTION The concept of developing building emulators@ a simulated building shell and simulated heating@ ventilation@ and air-conditioning (HVAC) equipment combined with real building automation and control system hardware@ as a tool for studying building control system performance emerged in the late 1980s. Early work led to an international collaborative effort to explore variations of building emulator designs and applications in Annex 17 of the International Energy Agency@ Energy Conservation in Building and Community Systems program (Kelly and May 1990; Haves et al. 1991; Vaezi-Nejad et al. 1991; Karki 1993). Building emulators have found limited use as research tools@ training aids for control systems users@ and for control system performance evaluation. (Liebecq et al. 1991) and (Kaerki and Lappalainen 1994) designed prototype emulators to test such aspects as accuracy@ time-step@ zone temperature control changes and tuning loop parameters. (Larech et al. 2002) developed a test method for evaluating HVAC controllers by emulation. (Brambley et al. 2005) discuss emulation for training@ FDD@ operational strategizing@ and optimal control and state that ""Computer emulation of building conditions that are fed into controllers will speed the adoption of new technologies by providing a resource for testing controller hardware under a complete range of conditions."" Building emulators vary in design details but common characteristics include real-time simulation linked to a hardware interface that couples the simulated building and simulated mechanical equipment to the controllers. Digital-toanalog converters are used to convert simulated sensor information such as temperatures@ pressures@ and flows into electrical signals that are wired to the sensor input terminals of the control hardware. Analog-to-digital converters are used to convert analog control signals into digital values that are fed into the simulation. Digital inputs and outputs are used for switching and status signals. The overall effect from the perspective of a building controller is that it ""thinks"" it is receiving real sensor input and controlling real building equipment; but@ in reality@ the sensor data and equipment are simulations. A building emulator combines the reproducibility and flexibility of simulations with the real performance constraints of actual control hardware. The development and widespread use of the BACnet communication protocol standard (ASHRAE 2008; Bushby 1997) combined with rapid advancement in the capabilities of computer control hardware for building applications has made possible a new generation of ""cybernetic building systems."" Cybernetics is the science of communication and control theory that is concerned especially with the comparative study of automatic control systems (Webster's@ 2009). A cybernetic building integrates building automation and control systems for comfort control@ energy management@ and fire detection@ security@ and transport systems. It also integrates the building systems with outside service providers and utilities. Cybernetic building systems offer the potential for significantly more energy efficient building operations@ lower maintenance costs@ and improved occupant comfort and safety. The current generation of mechanical systems used for heating and cooing and their associated building automation and control systems almost never achieves their design efficiencies at any time during building operation and their performance typically degrades over time. For example@ case studies indicate that energy consumption for HVAC systems can be reduced 20% just by detecting mechanical faults and ensuring that systems are operated correctly (TIAX LLC 2005). Additional case study examples can be found in Ardehali et al. (2003). The vision of cybernetic building systems involves a much more complicated web of potential building system interactions and interactions between building systems and external entities such as utility providers than is typically found in buildings today. In order to achieve the potential of cybernetic buildings there is a need to understand the failures of today's systems and ways to reliably take advantage of new opportunities that system integration provides. Buildings are complex systems of interacting subsystems. Most commercial buildings are ""one-off"" designs with unique operating needs. Interactions between subsystems can be complex and are often not well understood. The industry is very sensitive to the first cost of new technologies and performance goals such as energy efficiency@ indoor air quality@ and comfort often conflict. There are no simulation tools that can realistically capture all of the necessary details of a complex cybernetic building system. An emulator is needed that can combine the strengths of simulations with the constraints of actual commercial control hardware and communication technology. A real controller has constraints in memory@ processor speed@ the number and size of the registers@ the operating system features@ and the design choices made when creating the control algorithms. These constraints are very important when trying to test and demonstrate the feasibility of algorithms that are intended to be embedded in the controllers."