Importance

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Abstract

A data transmission system causes a remote data set to dial a central computer periodically and also causes the data set to stand by at more frequent intervals to detect a call from, and then return a call to, the computer via a non-dedicated telephone line. The system is coupled to a telephone instrument, a data set, and the telephone transmission line and normally connects the telephone instrument and the data set to the telephone transmission line. A clock in the system triggers the data set to call the central computer periodically, as for example once a month. More frequently, as for example during one hour each night, the clock connects the telephone instrument and the data set to a subsystem which stands by during the interval to detect a possible call from the computer. During the interval, the telephone instrument is normally disconnected from the telephone transmission line and the data set remains connected by the subsystem. A counting circuit senses the number of ring bursts in a ringing signal from a caller and the central computer, if it needs to access the data set, is programmed to ring once and 'hang up'. When only one ring burst is sensed, the data set dials up the central computer. If more than one ring burst occurs, a non-computer call is being sensed and the data set is disconnected and the telephone connected to the telephone transmission line so that normal telephone usage is allowed.

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First Claim