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In telephony, ringdown is a method of signaling an operator in which telephone ringing current is sent over the line to operate a lamp or cause the operation of a self-locking relay known as a drop. Ringdown (a) is used in manual operation, as distinguished from dialing, (b) uses a continuous or pulsed ac signal transmitted over the line, and (c) may be used with or without a switchboard. The term ringdown originated in magneto telephone signaling in which cranking the magneto in a telephone set would not only ring its bell but also cause a drop to fall down at the central office switchboard, marked with the number of the line to which the magneto telephone instrument was connected. The last ringdown telephone exchange in the United States was located at Bryant Pond, Maine, had 400+ subscribers, and converted to dial service in October 1983.
In an application not involving a telephone operator, a two-point automatic ringdown circuit, or ringdown, has a telephone at each end. When the telephone at one end goes off-hook, the phone at the other end instantly rings. There is no dialing and phones without dials are sometimes used.
Many ringdown circuits work in both directions. In some cases a circuit is designed to work in one direction only. That is, going off-hook at one end (end A) rings the other (end B). Going off-hook at end B has no effect at end A.
In some circumstances, the electronics that operate ringing are part of a key service unit. This is the electronics box that operates a multi-line key telephone system. In the wire spring relay vintage Bell System 1A2 key service units, a model 216 automatic ringdown was used to operate the circuit. In the 400-series units, there are a number of different KTUs that operate (supervise) a ringdown including a 415. In other situations, the ringdown is powered and operated by equipment inside the central office.
In the case of enterprises with a PBX switch, the ringdown can be operated by the PBX switch. The switch is programmed to ring a specific extension (the called phone) when a defined extension (the calling phone) goes off-hook. The PBX does not offer dialtone to the calling extension: it only looks for on-hook or off-hook status.
These circuits are used:
Example: an information desk and the information desk staff supervisor's desk.
Example: a phone used to summon a taxicab to an airport or hotel.
Example: the after-hours phone to reach the watchman from the front door at a warehouse.
Example: an airport control tower to the airport's fire station or fire dispatch center.
Example: Independent System Operator (ISO) communication to a power plant.
Example: a hospital emergency department and an ambulance dispatch center.
In some cases, automatic ringdown circuits have one-to-many configurations. When one phone goes off-hook, a group of phones is made to ring simultaneously.
In cases where one or both ends of the circuit terminate in a key telephone system, a well-designed system will have no hold feature on the ringdown circuit unless supervision provides a Calling Party Control (CPC) signal.
PLAR stands for Private Line Automatic Ringdown. It is a type of analog signalling often used with telephone based intercom systems. When a device is taken off-hook, it applies a ringing voltage to the circuit. Other devices on the same pair will ring. Then, when another device is answered, a call will be maintained over the circuit at normal voltage. The telephone company switch is not involved in the process, making this a private line.