Patent GB-1513527-A (GB)
Title: Operating condition data communication system
IPC: H04M11/04
Filed: 1976-01-27
Abstract: 1513527 Selective signalling TULL AVIATION CORP 27 Jan 1976 [3 Feb 1975] 03040/76 Heading G4H In a data communication system, a central station can establish a connection over a non- dedicated voice band-width communication link to a passive station for transmitting commands to it and receiving responses, the passive station including means operable whenever a connection is established with the central station to issue a ready signal to the central station inviting it to issue a command to the passive station, the passive station including means to initiate a call-out operation for establishing communication over a non-dedicated voice band-width communication link to the central station in response to detection of an alarm condition at the passive station, the capability of the passive station to establish communication with the central station being limited to the establishment of the connection and the transmission of a ready signal to the central station inviting it to issue a command to the passive station. As disclosed, the central station includes a computer (or keyboard), a dial-out unit and a modem, and can communicate via the telephone system (including one or more exchanges) with the passive station which may be one of a number and which controls and collects data from a number of navitational aid transmitters at an airport, directly in the case of one transmitter and via dedicated links and satellite stations in the case of others. The passive station includes a dial-out unit, modem, digital voltmeter (fed via a multiplexer from the transmitter and a transmitter-monitoring unit, and feeding storage), and a status register for alarm conditions. When in communication with the central station, the passive station excutes commands received as they arrive and after each sends back a signal confirming execution of the command followed by the ready signal to invite the next command. Some commands ask for data to be sent to the central station, from the storage (holding the digital voltmeter readings) or the status register (in response to alarm calls), or ask for a satellite station to be connected to the central station. The passive station includes a number of timers respectively : (a) to reset and to send the ready signal if there is a 40-second gap in data sent from the passive station to the central station, (b) if a satellite station is in communication with the central station but sends no information for 20 seconds, to cause the passive station to seize the link with the central station (the central station being notified of the change since the passive station's ready signal, which it now receives, differs from the ready signals of the satellite stations), (c) to disconnect the passive station from the telephone line after an interval somewhat longer than the usual interval for which communications will normally continue between the central and passive stations. Each satellite station includes a status register for alarm conditions thereat. An alarm condition at a satellite station causes an alarm call to the central station via the passive station and the status register of the passive station. The transmitter-monitor unit may give an alarm signal to set a stage of the status register, cause the multiplexer and digital voltmeter to enter measurement results in the storage and shut down the transmitter, then after a delay, restart the transmitter and if the alarm condition persists, set another stage of the status register, store measurement results again, shut down the transmitter and make an alarm call. A "call originate" counter is provided in the passive station to limit the number of successive unsuccessful attempts to call the central station, a timer being used to specify an interval no replay within which will be taken to indicate an unsuccessful attempt. Alarm conditions include: intruder (door-actuated switch), power failures, transmitter frequency out of permitted range. Data is sent using audio fsk.