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

HVAC Control in the New MillenniumA basic network is comprised of a master such as a PC or PLC anda set of simple devices such as on/off switches or motor starters.Object-oriented techniques (OOT) are used where objects are definedin class, instances and attributes. This allows changes to be mademore easily. Four basic objects are used. An identity object providesinformation about the identity of a device. A connection object containsthe number of logical I/O ports of the device.A DeviceNet object configures the port attributes such as the nodeaddress and data rate. It maintains the configuration and status of physicalattachments. You can remove devices from the network or insertthem into the network without removing network power. You can alsoadd power taps at any point in the network and have redundant powersupplies. One goal of DeviceNet was to achieve the same level of interchangeabilityfor 120/220V-ac and 24V-dc discrete devices using digitalcommunications.DeviceNet provides interchangeability of simple devices whilemaking interconnectivity of more complex devices possible. Instead of asource-destination approach, DeviceNet uses the producer/consumermodel, as provided by CAN, where each data packet has an identifierfield. The identifier provides for multiple priority levels for arbitrationand multiple consumers.The device with data produces the data on the network with theproper identifier and all devices needing data listen for messages. Whendevices recognize the appropriate identifier, they consume the data.The message length for DeviceNet is 0-8 bytes, which is idealfor devices with small amounts of frequently exchanged data.DeviceNet supports strobed, polled, cyclic, change-of-state and application-triggereddata movements. Communication can be master/slave, multimaster with different slaves, peer-to-peer, or a combinationconfiguration depending on device and application requirements.Response time for a network of up to 64 nodes and 500 I/O can beless than 5-ms.Devices can be powered directly off the bus and communicateusing the same cable. Nodes can be configured, removed from, or insertedto the network without powering down. Power taps can be addedat any point in the network allowing redundant power supplies.Trunk and drop lines can use thick or thin cable. The networkdistances depend on data rate and cable size. A range of over 500 feetis possible.©2001 by The Fairmont Press, Inc. All rights reserved.