1072 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



cause a 900-cycle pulse to be sent back to the alarm center for each indica- 

 tion scanned. Whenever an alarm condition or other indication is encoun- 

 tered, a 700-cycle pulse is transmitted simultaneously with the 900-cycle 

 pulse. At the alarm center the pulses are selected by 700- and 900-cycle 

 filters, and amplified and detected in the same manner as the station tones. 

 The resultant d-c. pulses operate relays which in turn light particular lamps 

 in the key shelf lamp display panel in the alarm center receiving bay when- 

 ever the two pulses are received simultaneously, 



C. Sequence Signaling Remote Controls 



As mentioned earlier the CI alarm and control system is capable of trans- 

 mitting as many as ten orders from the alarm center to a particular station 

 in trouble. Typical orders to a repeater station may be an order to scan all 

 alarm indications or an order to start the gas engine alternator. Sequence 

 signaling transmitters and receivers are employed for the transmission of 

 orders to the unattended repeater stations. Sequence signaling is an ar- 

 rangement in which two separate signals sent in a predetermined sequence 

 are translated by the receiver into an order. One hundred and thirty-two 

 different orders can be transmitted from an alarm center through sequence 

 combinations of two out of twelve modulating frequencies available in 

 15-cycle steps from 277.5 to 442.5 cycles. The CI system makes use of 120 

 of these orders at those alarm centers which remotely control as many as 

 twelve unattended repeater stations. 



An attendant initiates an order by operating the key of the station to be 

 called, the proper order key and a start key. This operation selects the proper 

 two low frequencies which modulate a 1600-cycle carrier oscillator and the 

 sequence in which they are sent. The incoming signal to the sequence signal- 

 ing receiver at an unattended station is amplified and demodulated. The 

 two low-frequency tones recovered from the 1600-cycle carrier are applied 

 sequentially to the receiving director. The director identifies the tones by 

 means of four accurately tuned reed selectors, recognizes their sequence and 

 translates them into one of ten orders for that particular repeater station. 



VIII. Power Equipment* 



The TD-2 System is supplied by battery voltages of -12, -f- 130 and +250 

 volts maintained by charging rectifiers which float the batteries within 

 limits of ± 1%. A 24 volt battery to supply power to the CI alarm and order 

 wire circuits is also included in the power plan where necessary. The block 

 diagram illustrated in Fig. 22 shows the inherent simplicity of the plant. 

 During power failures the batteries carry the load until an automatic gas 



* Prepared by J. M. Duguid. 



