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.
This led to the phrase "ring off" meaning to finish a telephone conversation.
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:
- over high-volume routes where one site calls another very frequently.
Example: an information desk and the information desk staff supervisor's desk.
- where a tamper-proof ability to call from one point to another is needed.
Example: a phone used to summon a taxicab to an airport or hotel.
- where the public, or users that are not trained in using a specific office telephone system, must place calls.
Example: the after-hours phone to reach the watchman from the front door at a warehouse.
- in locations where emergencies are handled and the time required to dial digits would cause an unacceptable delay in handling of an emergency.
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.
- in situations where the called party needs to be certain of who is calling.
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