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

HVAC Control in the New MillenniumThe peripherals respond to the poll from the host, and do not initiatetransmission. The peripheral devices do not transmit unless they aregiven response permission.An exception is when the host is in sleep mode and a peripheralinitiates a wake-up call for service. Then the host steps back into thepolling sequence and looks for devices to service. If there is not anytransmission between the host and any peripheral for a set time, the hostagain enters the sleep mode. Hosts do not communicate with each other.If multiple hosts are used, they detect each other and dither their transmissionsto reduce the chance of interference.The requirements for IrDA Control communication vary dependingon the application. The sleep mode is used to minimize power consumptionwhen a host and its peripherals do not need to communicate.The normal operational mode supports peripherals that may have differentbandwidth requirements.PagersAlphanumeric pagers are used for short messages. They usuallydisplay a few words or the caller’s number. For lengthier messages, youcan plug a PCMCIA device into your notebook or PDA to receive wirelesse-mail. PCMCIA pagers can serve as pocket-size pagers and theycan be plugged into a notebook when you want to read and composelonger messages.Wide-area RF equipment may utilize one-way and two-way dataflow. One-way data flow is the usual technology that is used for digitalelectronic pagers. In a one-way system, a carrier service, such as SkyTel,transmits a message and an address (the pager ID) over the geographicarea it serves.The pager recognizes its ID, then receives and displays the incomingdigital message. Two-way service is similar except that the receivingunit also sends data back to the carrier, which forwards the data toanother computer on the LAN. There are also PCMCIA cards that canreceive and hold pager calls.Wireless RF modems can be used to connect portable computers toa LAN. A cellular modem or modem/adapter that uses analog audiosignals to convey digital information is called a voice-band modem or ananalog cellular modem.©2001 by The Fairmont Press, Inc. All rights reserved.