In the past few years there has been considerable discussion regarding computerized control of heating, ventilating, and air-conditioning systems. Most of this discussion has centered on com-puter hardware and software. Interfacing with the local control loop has been overlooked largely because the methods employed by the numerous manufacturers and contractors have been very similar.

With the adoption of the Inter-Agency EMCS guide specification, there has been a proliferation of building automation systems being constructed. The designers and contractors which have been drawn to this field have brought with them alternate, and often less costly, methods of interfacing with the local control loop.

This paper presents a very basic overview of the method of effecting two-position and proportional control of various devices, (e.g., value operators and damper operators). It also presents the considerations involved in interfacing with motor starters. While alternate methods are offered, the traditional approach is detailed for those who have no prior exposure to automation systems.