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

Computer Networks and SecurityIn DeviceNet’s communication stack of OSI layers, the CAN protocoldefines the physical and data-link layers. DeviceNet adds the remainderof those layers, along with the media and application layers.A DeviceNet network can have as many as 64 node addresses.Since it uses the CAN data-link layer, DeviceNet is a producer-consumermodel, which requires packets on the network to include data-identifierfields. This approach is in contrast with older technologies where messagesinclude a explicit source and destination. The identifiers tell thedevices if they should consume the data.The network assigns transmissions associated with a connectiondevice configuration. The messaging protocol indicates how a deviceshould interpret a message. A message router transfers received messagesto target objects. The priority of access depends on the MessageGroup the device belongs to. Device configuration takes place throughthe device connections when the device identifies it.DeviceNet Objects and DevicesThe object model provides a template for organizing and implementingthe attributes (data) and service (methods or procedure). Anidentity object’s attributes include vendor ID, device type, product code,revision status and serial number.To facilitate compatibility and interoperability, DeviceNet definesstandard device profiles, designed to allow you to interchange vendorsand products that conform to the same device-type profile.DeviceNet provides a communications link for connecting devicessuch as sensors and motor starters to a network. DeviceNet uses twotwisted pairs of wires to provide 24-VDC power and signal. One surveyby Venture Development Corporation found that DeviceNet was usedby 44% of instrument and control device suppliers.DeviceNet FutureU.S. device/sensor buses are projected to grow to 2.7 million nodesin 5 years. DeviceNet should account for over one-third of this total.More than 300 ODVA members direct the future of DeviceNetthrough special interest group activities. These include alternative media©2001 by The Fairmont Press, Inc. All rights reserved.