Abstract: An electronic key telephone line circuit is disclosed for controlling the hold and disconnect conditions with respect to associated telephone stations in response to transient signals generated upon on-hook to off-hook and off-hook to on-hook transitions of those stations. Due to the nature of the switchhook contacts, the tip and ring leads open before the A-lead opens. Since the disconnect condition is determined from a transient signal caused by the tip and ring lead opening together with an A-lead open, and since by its very nature the transient response of the tip and ring lead transition can be finished before the A-lead opens, timing is used to delay the transient signal to insure that the line circuit goes into the proper mode.

Abstract: There is disclosed a circuit for generating tone signals from clocking signals present on each key telephone line circuit. The tone signals are transformer coupled onto the communication leads by an optional plug-in-board arrangement. In present installations music-on-hold is provided by connecting an external music source to each line card. There is also disclosed an arrangement for utilizing the optional plug-in-board to provide either tone-on-hold or music-on-hold by simply reversing a plug-in connector.

Abstract: There is disclosed a circuit for providing various lamp signals to a line lamp of a key telephone station. The circuit is arranged with a single ac switch or triac for each line circuit. The triac is driven at the desired rate by a circuit which generates a series of phase-shifted digital zero crossing signals having the desired duty cycle rate. Advantage is taken of the fact that the triac only requires energization at the zero crossing of the ac signal.

Abstract: There is disclosed an electronic key telephone line circuit having a communication path interface capable of being connected across the communication pair during both the busy and idle conditions of the line circuit. The circuit is arranged such that both the pre-ring test and the impedance test from the central switching machine are satisfied while at the same time presenting a linear impedance for communication signals. Opto isolators are used and coupled with lightning surge protection and a capacitance and resistance bridge to allow permanent connection across the communication pairs. The circuit includes a hold bridge impedance, separate from the signal detection circuitry, switchable across the communication path.