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

HVAC Control in the New Millenniumtors, and electrical characteristics. The data-link layer defines the formatfor an Ethernet frame, the error-checking method and the physical-addressingmethod.The data-link layer also defines the protocol Ethernet uses to determinewhen nodes can transmit on the network. This is known as carriersense, multiple access with collision detection. There is a need for standardupper-level protocols to be used with Ethernet.Ethernet is only one of the protocols needed to make any LAN operational.Ethernet cannot help a message find its way through a complexnetwork. It cannot define how to carry out specific tasks on a network,such as file transfers. These operations require additional protocolson top of Ethernet to create a protocol suite.In Ethernet there is no assurance of when a node can get on thenetwork. If the network is busy, then a node with a message may haveto wait. The length of the wait would be based on probabilities. Thismakes Ethernet nondeterministic.In a proportional-integral-derivative (PID) control system, it couldbe a problem. The collection of sensor data over an Ethernet segmentshould have a regular sample time to be effective. In theory, Ethernetmay not be able to guarantee consistent timing.The determinism issue would have you believe that any Ethernetnetwork loaded at about 40% would encounter an exponential growth intransmission delay times. But, studies conducted in the late 1980sshowed that, in practice, Ethernet delays tend to be linear and can beheld to less than 2 milliseconds for a lightly loaded network and 30milliseconds for a heavily loaded network. These delays are acceptablefor most HVAC control applications.The main concern is to keep the traffic level on the Ethernet networklow enough so that collisions occur at low frequency levels. Thismeans restricting the number of devices and the amount of traffic thateach device generates so that the total segment traffic never rises above1 or 2%.If more devices are needed, they are put on separate segments andthen connected using a bridge, router, or switch. This concept is used byDCS systems such as ABB’s MasterBus and Foxboro’s Nodebus.Besides the commercial-grade hardware for Ethernet, severalmanufacturers offer industrial hubs and switches that can be DIN-railmounted and have redundant power supplies. Ethernet is easy to installand connect.©2001 by The Fairmont Press, Inc. All rights reserved.