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

Building Automation, Wireless Technology and the InternetThe transceiver is an analog device which converts electrical and opticalsignals and uses the following parts:• Digital-to-analog converter (DAC),• Analog-to-digital converter (ADC),• Preamplifier, and• Gain-control circuitry.In the protocol stack, each layer has certain responsibilities. ThePhysical Layer specifies the optical characteristics, encoding scheme andframing definition. The Link Access Protocol establishes the basic connection.An object- exchange protocol which enables the transfer of filesand other objects. There is also serial and parallel port emulation andlocal area network access for notebook computers and other mobiledevices.The IrDA Control specification defines the transmission speeds,modulation schemes, and infrared (IR) wavelengths of the optical signalsemitted by the transmitter as well as those signals received by thereceiver.An encoder and decoder reside in the bitstream path and handledata coding as well as the modulation process. Data are passed from thecontrolling device to the encoder/decoder and then on to the transceiver.A 1.5-MHz subcarrier process and the coding of the transmissionsignals are used to minimize interference with other transmission systems.When an action is to be completed, the controller makes a decisionand sends data out through the IR link to the other device. When polledby the host, the device responds, informing the host that it has informationto send. The host then requests the information and the devicesends it.The controller passes the data on to the modem function, whichhandles the coding and modulation. The transceiver performs the electrical-to-opticaltranslation between systems.Two types of packets are used in the IrDA Control system shortpackets and long packets. The 1.5-MHz subcarrier pulses in the data bitsare transmitted for a logical “1” and are not transmitted for a logical “0.”The host manages its communications with multiple peripheralson a time-division basis, using polled-response handshakes. The hostpolls all of the peripherals to determine which items need to be serviced.©2001 by The Fairmont Press, Inc. All rights reserved.