Book - School of Science and Technology

telephone link. ISDN links may be used on large or complex sites. Increasingly `headends' can be viewed as web pages to allow connection via corporate intranets.All BEMS cabling should be protected from extraneous electrical interference and fibreoptic technology may be used for data transmission.OutstationsProcessors located close to plant, capable of handling a high volume of signals fromdifferent components. With increasingly cheap processing, the definition of outstation ismoving towards that of individual intelligent networked controllers (say, serving oneAHU).Sensing devicesFor every point to be monitored, a sensing device; switch; auxiliary contact or the like willbe required.Data inputsDigital ± two position switching signals for status and alarm.Analogue ± values measured by varying voltage or current in the form of a pulsedsignal.Control ± signals to component driving mechanisms.KeyboardA means to input instructions.Building management systems 655PrinterAn output device for permanent record of alarms; status; trend logging, etc.ScreenA screen display of system schematics or a summary of status; can give instantaneousvalues (dynamic display).Keyboards, printers and screens may be duplicated at different locations around abuilding; typically in the fire control centre (where provided), in the engineer's office, inthe maintenance department and at a central security station (for out of normal hoursmonitoring). Different levels of access into the system would be available to limit thefacilities appropriate to the user code. An example of an installation network is shown inFigure 22.34 to show the disposition of terminal devices.SoftwareAll systems should run under a graphical interface (although the ability for experiencedusers to use text commands may be an advantage on large systems). These computerprograms set the logic, procedures and rules concerning the operation of any dataprocessing system. Becoming increasingly important is the need to connect togethercomponents and systems by different manufacturers. Although the merit of using thesame transmission system for both control functions and life safety systems is in somedoubt, there is considerable benefit to be gained from controls manufacturers using acommon protocol standard. Integration between different systems, for example BEMSand lift, security and fire alarm systems, can either be by the use of a common dataprotocol or by the use of a gateway access which in effect is a `black box translator' whichenables transfer of signals between the different systems.