Non-operator Use
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.
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.
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