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

HVAC Control in the New Millenniumceiver are often bundled together into a single personal computeradapter board called a network interface card (NIC) or a multimediaaccess unit. Transceiver and controller functions are specified by the OSImodel. Table 7-6 lists thetransceiver functions.The transceivers limitaccess to one caller at a timeand direct the transmission.First, the transceiver listensfor a quiet period. When thenetwork cable is not busywith another transmission,the transceiver transmits thedata signal.Two or more transmittersmay simultaneously observea quiet period andtransmit. This results in a collision.Then, the transmittingtransceivers will broadcast ajam signal. Upon receipt ofthis signal, each station contributingto the collision must wait for a random period of time beforeattempting to transmit again. Different mechanisms are used for collisiondetection. These differences make some hardware incompatiblewith other hardware.The Ethernet controller dials the number and builds the transmittedpacket which is addressed to a designated node. The controller alsodisassembles packets and transfers this information to the receivingcomputer.The network access unit (NAU) also provides some services toincrease reliability. The NAU shuts down when it senses that thetwisted-pair wire is connected to a live telephone circuit. It also tests theconnections.Transmission ProcessTable 7-6. Transceiver Functions———————————————Receive signalTransmit signalBroadcast jam signalTest for receptionTest for transmissionTest for jam (collision)Test for heartbeatTest for carrier senseTransmit preambleTransmit delay signalSense collision———————————————Baseband Ethernet transmits a single digital channel. A base voltageof + 0.7 V provides a carrier sense signal for the nodes on the net-©2001 by The Fairmont Press, Inc. All rights reserved.