DDC: SOFTWARE AND HARDWARE.

This article has had a two-fold purpose: to survey the various DDC interface and software algorithm designs utilized by EMS manufacturers, and to provide enough background to the reader to assure an understanding of the difference between them. It was shown that the two obvious categories of DDC - pneumatic and electric - shed little insight into the major design variations. The most important classifications for DDC interfaces are the positioning and integrating types. This design choice affects the PID algorithm required, the response of the controller, certain control options and may even make control difficult. Namely, the positioning interface requires the positional PID algorithm. The positional algorithm requires an anti-windup feature. The response of an integrating interface is restricted by the speed at which the interface can change its output. An integrating interface cannot be easily used for selecting an actuator position. Lastly, the solenoid air valves interface, in its basic form, is nonlinear and will not operate well with a conventional, linear PID algorithm.