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

HVAC Control in the New Millenniumoptions such as noninsulated flatwire cable, to reduce costs, and gatewaysand high-speed routers to other networks.When two or more nodes try to access the network simultaneously,an arbitration scheme resolves the possible conflict with no loss of dataor bandwidth. This is much different than Ethernet where collision detectionis used where a loss of data and bandwidth can occur since bothnodes have to back-down and resend the data. If two nodes transmitsimultaneously, the winner is the one with the lower numbered identifier.Cyclic redundancy checks are used by CAN controllers to detectframe errors. Automatic retries are also used. These methods are usuallytransparent to the application and serve to block a faulty node fromupsetting the network.CANThe controller area network (CAN) was developed by Bosch toreplace expensive harness wiring. The technology of the CAN bus wasdevised in the mid-1980s as an in-vehicle network for automobiles. Sincethe 1980s, the automotive industry has been using electronic controlsystems for in-vehicle functions and features that improve safety andcomfort.In a system with conventional wiring, data are exchanged usingdedicated signal lines. As the complexity and number of devices increased,the use of dedicated signal lines became increasingly difficultand expensive to implement.CAN is used as the Data Link Layer of DeviceNet. CAN controllerchips allow data to be moved on DeviceNet using a data frame. CANchips are available from Phillips, NEC, Intel, Siemens and NationalSemiconductor. A 0-8 byte size is used for low-end devices where smallamounts of I/O data that must be exchanged regularly. Simple devicescan also send diagnostic data.Bus systems like CAN overcome some of the limitations of conventionalwiring. Using the CAN bus, devices are connected to a commonserial bus. Any device on a CAN bus network can communicate withany other device using a common pair of wires.CAN bus features include CAN frames which have identifiers atthe beginning of the frames. This identifier is used to identify which©2001 by The Fairmont Press, Inc. All rights reserved.