Telephone switchboard

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A switchboard (also called a manual exchange) was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges (PBXs). The user was typically known as an operator. Public manual exchanges disappeared during the last half of the 20th century, leaving a few PBXs working in offices and hotels as manual branch exchanges.

The electromechanical automatic telephone exchange, invented by Almon Strowger in 1888, gradually replaced manual switchboards in central telephone exchanges. Manual PBXs have also for the most part been replaced by more sophisticated devices or even personal computers, which give the operator access to an abundance of features. In modern businesses, a PBX often has an attendant console for the operator, or an auto-attendant avoiding the operator entirely.



The switchboard is usually designed to accommodate the operator to sit facing it. It has a high backpanel which consists of rows of female jacks, each jack designated and wired as a local extension of the switchboard (which serves an individual subscriber) or as an incoming or outgoing trunk line. The jack is also associated with a lamp.

On the table or desk area in front of the operator are columns of keys, lamps and cords. Each column consists of a front key and a rear key, a front lamp and a rear lamp, followed by a front cord and a rear cord, making up together a cord circuit. The front key is the "talk" key allowing the operator to speak with that particular cord pair. The rear key on older "manual" boards and PBXs is used to physically ring a telephone. On newer boards, the back key is used to collect (retrieve) money from coin telephones. Each of the keys has three positions: back, normal and forward. When a key is in the normal position an electrical talk path connects the front and rear cords. A key in the forward position (front key) connects the operator to the cord pair, and a key in the back position sends a ring signal out on the cord (on older manual exchanges). Each cord has a three-wire TRS connector: tip and ring for testing, ringing and voice; and a sleeve wire for busy signals.

When a call is received, a jack lamp lights up on the back panel and the operator responds by placing the rear cord into the jack and throwing the front key forward. The operator now converses with the caller and finds out where the caller would like to be connected to. If it is another extension, the operator places the front cord in the associated jack and pulls the front key backwards to ring the called party. After connecting, the operator leaves both cords "up" with the keys in the normal position so the parties can converse. The supervision lamps light to alert the operator when the parties finish their conversation and go on-hook. When the operator pulls down a cord, a pulley weight behind the switchboard pulls it down to prevent it from tangling.

On a trunk, on-hook and off-hook signals must pass in both directions. In a one-way trunk, the originating or A board sends a short for off-hook, and an open for on-hook, while the terminating or B board sends normal polarity or reverse polarity. This "reverse battery" signaling was carried over to later automatic exchanges.

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