s emissions 0.25 W (max.) CTCSS (optional) To EIA standards RS220 frequencies 103.5, 114.8, 151.4, 167.9, Figure 45.2 A fully featured on site paging system 225.7, 250.3Hz etc. For direct machine driven paging calls, or push button driven paging calls, a remote contact interface is used. Typical uses are for callers who require entry to a locked building patrolled by a security Modular systems such as these may have as few as 15 or as many guard carrying a pager, or by process engineers who are supervising as 1500 pagers. The common bus connecting these units forms a many pieces of continuous running equipment, and need to be local area network with a bus cable length of up to 1km. The bus notified of faults, or important machine parameter changes. cable is 8 way, with one pair of data lines (RS485 standard), 1 pair Connection to a standard computer by RS232c line is shown. The outgoing speech, 1 pair return speech and the remainder used for computer operator could also page by directory viewing if this earth connection. The data rate is 19.2kbit/s and digital data voltages software is entered into computer memory. are 0 and +5V. For the speech pairs frequency band is 300Hz to The call logging facility provides a record of times and user 3kHz at a line level of -13dBm. numbers actually called on the system, which is useful when re- The master/slave architecture, where one module is used as the viewing system usage. master and the rest of the modules are treated as slave devices, Absence rack units automatically register the user as absent, when operates a communications protocol using a token passing scheme. he places his pager in one of the slots. Subsequent callers will be This scheme gives permission for slave devices to use the system informed that he is unavailable or the call may be transferred to an data bus on command of the master. The master also holds a device alternative pager. These are usually sited at staff exits and usually parameter table, which stores the initialisation parameters for every cont