1070 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



order circuit is used to link the terminal stations of a system with the main 

 stations, alarm centers and maintenance centers for system-wise radio 

 maintenance and traffic control. 



A. Station Alarms 



Each unattended station transmits a continuous and distinctive tone to 

 its associated alarm center. Interruption of this tone for approximately ten 

 seconds registers an audible and visual alarm at the alarm center, and, be- 

 cause each station is assigned a different frequency, the station whose tone 

 is interrupted is easily identified. As many as six stations can report to an 

 alarm center over a one-way, two-wire alarm line. Six more alarm sending 

 stations can report to the same alarm center by bridging them on a second 

 alarm line (usually in the opposite direction from the first), and providing a 

 second set of receiving filters with associated amplifiers and detectors at the 

 alarm center. In this manner an alarm center can identify the alarms from 

 a maximum of twelve unattended repeater stations. The six station fre- 

 quencies that can be used on one alarm line are 1100, 1300, 1500, 1700, 1900, 

 and 2100 cycles. 



Each station alarm tone is selected at the alarm center by its associated 

 receiving filter and individual amplifier-rectifier circuit. Automatic gain 

 control action in the amplifier circuit permits a tone from the alarm line 

 to vary ±6 db from its normal value without interfering with the proper 

 operation of the system. 



B. Individual Alarm Indications 



The station alarm reports that a particular unattended point is in trouble, 

 but it does not tell what the specific trouble is. Supplementing the station 

 alarm circuit is an individual alarm indication circuit that reports which, 

 if any, of 42 possible alarm conditions exist at an unattended station. The 

 alarm indication sending circuit does not start automatically but only in 

 response to an order sent out from the alarm center. Thus, after receiving a 

 station alarm, an attendant at the alarm center sends an order over the 

 control system described later to that particular station directing it to scan 

 the individual alarms and report those that have operated. 



The report is transmitted over the alarm pair and received on a miniature 

 lamp bank located in the key shelf of the alarm receiving bay at the alarm 

 center. Of a total of 60 lamps in the key shelf, 42 are used for alarm indica- 

 tions, 8 for identifying the six east or west reporting stations, and 10 for 

 checking synchronization of the indication sending and receiving circuits. 

 Figure 21 is a copy of the form which is placed over the lamp display to 



