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

Computer Networks and Securityavailable to provide the diagnostics needed. In the display end, efficientinterfaces and software with extensive template facilities should beused.The concept of an industrial control network is to link all devicesor stations with one cable. No matter how many devices you are connecting,you just add the physical cable which is needed to connect tothe furthest station. Troubleshooting becomes much easier since you canplug and unplug the device from the network. This virtually implies aplug-and-play ability of the devices.The best of buses for one industrial automation application may bethe worse of buses for another. Wide-ranging requirements have spurredthe creation of specialized networks.The move to open digital networks marks a significant change inthe architecture of control systems. As with any major technologicalchange, much contention surrounds the choice of technologies, methods,and architectures.The IEEE-488 standard has been the dominant communicationsinterface for electronic instrumentation for more than 20 years. IEEE-488has evolved to include more software standardization and increasedtransfer rates. It continues to dominate test-and-measurement applications.Several major technological shifts have occurred in control systemsover the last half century. Process control has shifted from manual systemsto pneumatics, followed by electronic control systems based on the4- to 2-mA instrumentation standard. Many distributed control systemsstill use 4- to 2-mA instrumentation wired to I/O subsystems, which inturn are connected to larger systems by proprietary digital networks.The most significant move in the early 1970s was from hardwiredrelay panels to programmable logic controllers (PLCs). These PLC-basedsystems directly wired the analog and digital signals to I/O subsystems,which were connected to the PLC using vendor-specific digital networks.Devices that include both digital-communication and sensing capabilitiesdistinguish today’s digital networks. A common network thatdirectly connects the control system’s measurement and control devicesreplaces point-to-point wiring to centralized I/O subsystems. Digitalnetworks provide several benefits. One of their most basic characteristicsis the increased information available from devices that communicatedigitally.©2001 by The Fairmont Press, Inc. All rights reserved.