Abstract: A single line hold circuit adapted for connection between the tip and ring leads of a telephone line. The circuit comprises impedance means for connection between the tip and ring leads, the impedance means including a semiconductor element having high and low impedance states, the impedance of the low impedance state being sufficiently small to maintain the telephone line in a hold condition. The circuit further comprises means for causing the semiconductor to be in its low impedance state following an interruption of a predetermined length of time in loop current. A double line hold circuit is also illustrated.

Abstract: The system includes up to four communication links, a separate link selector for each link, and a separate link scanner for each telephone station. The system is suitable for usage with conventional dial pulse or tone selective key telephone sets.

Abstract: The system receives subscriber station address information in the form of frequency coded tone signals transmitted from a calling station along a common intercom circuit, and selectively accesses a called subscriber station having a corresponding station address in order to produce a call indication ring-out thereat. The system is comprised of decoder circuitry including a digital multi-frequency tone detector for receiving and decoding station address information transmitted from a calling station, and access circuitry operatively associated with the tone detector for routing an appropriate call indication signal to the called station. Signals which appear at the output of the tone detector are delivered to the access circuitry which, in turn, initiates the call indication sequence and routes additional signals back to the tone detector for controlling it during call indication.

Abstract: The system receives subscriber station address information in the form of dial pulses transmitted from a calling station along a common intercom circuit and selectively accesses a called subscriber station having a corresponding station address in order to produce a call indication ring-out thereat. The system includes a hook condition detector circuit which controls multiple system functions with respect to the on-hook and off-hook status of the calling station, and further includes a dial pulse detector circuit which effects additional control with respect to completion of dialing by the calling station. The system is suitable for use with ten or varying numbers of subscriber stations and may include, for applications having more than ten subscriber stations, appropriate transfer and associated output control circuitry for double digit operation.

Abstract: The system includes a central station console and one or more remote station modules. Corresponding message-waiting indicators mounted by the central station console and each remote station module are accessed and controlled from the central station on a selective individual basis; but can be controlled alternatively from the remote station modules in order to indicate message-received. Corresponding do-not-disturb indicators mounted by the central station console and each remote station module are controlled exclusively from the latter. For telephone application, the do-not-disturb indicators additionally may be operated in a flashing mode when the associated telephone set assumes off-hook condition in order to provide call status indication.