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

HVAC Control in the New MillenniumNetwork StructureThe AS-i network uses a tree structure with a maximum of 31slaves, to which 124 inputs and outputs can be connected. In most applications,the master is part of the first control level, but the network canalso be subordinate to a fieldbus system.The master becomes a slave in the fieldbus network. It controls andpolls the network, sending and receiving data from each connected device.It senses and registers any connected device without configurationor application-specific software. There are couplers for differentfieldbuses, including Profibus, Interbus, CAN, DeviceNet, and SDS.Over a million AS-i field devices have been installed. There areslave modules with integrated pneumatic valves, earth fault detectorsand slaves to handle analog signals. Repeaters and extenders can be usedto increase the 100-m network length.The number of slaves that can be connected to a network has beenincreased from 31 to 62. This is done with A/B addressing, which uses aspare bit in the protocol to switch between devices in succeeding buscycles. This increases the maximum network capacity to 248 inputs and186 outputs but doubles the cycle time to 10-ms.A spare bit in the status register is used to mark peripheral faults.The host can use a fast fault indicator, making it easier to detect faults ina network.HARTHART is a de facto standard in the automation industry. HART offersease of use, compatibility with existing systems, and cost-savingbenefits. HART retains the compatibility and familiarity of 4-20-mA systems,while providing remote two-way digital communication withsmart field equipment. HART is an important communication technologyused in smart instruments.Smart instrumentation is important in distributed intelligence andcontrol. The ARC Advisory Group has predicted that HART will dominatesmart-instrument communication. There is a major trend from traditionalanalog to smart instrumentation.The HART Communication Foundation (HCF) owns the technology,which was developed by Rosemount. HCF is a nonprofit organiza-©2001 by The Fairmont Press, Inc. All rights reserved.