HVAC Control in the New Millennium.pdf - HVAC.Amickracing

PC-based Control, Software and Bus Trends 173Chapter 5PC-based Control,Software and Bus Trends-based control is becoming a mainstream technology,PCyet there is still some controversy between programmablelogic controller (PLC) manufacturers and PCbasedcontrol suppliers on the use of PC-based controltechnology. PC-based control is much more than simply emulating aPLC on a PC. PC-based control systems using only traditional PLC programminglanguages realize only part of the technology’s real benefits.These benefits include the integration of the control system into a singletag database.This integration combines the functions of logic control, programming,operator interfaces, supervisory data collection and enterpriseinterface in a single platform. A common tag database among each of thefunctions eliminates multiple databases and the data communicationconfiguration needed for multiple components. Another major benefit isthe ability to use off-the-shelf PC hardware and software. PC-basedcontrol offers a lower-cost control solution and continuous efficiencyimprovements.Data connectivity between applications on the same or differentplatforms is possible through interfaces such as OLE for control (OPC),distributed communications and dynamic data exchange. OLE standsfor object linking and embedding.Object ConceptsAn object is a software concept that often embodies hardware elements.Objects are a more natural way of solving problems. Humansobserve and interrelate on an object basis.An object knows its purpose. It responds to stimuli (inputs), per-©2001 by The Fairmont Press, Inc. All rights reserved.