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E&M (earth & magneto, or ear & mouth) is a type of supervisory line signaling that uses DC signals on separate leads, called the "E" lead and "M" lead, traditionally used in the telecommunications industry between telephone switches. E&M was originally developed to allow PABXs in different geographic locations to communicate over an analog private circuit. Some digital interfaces such as Channel Associated Signaling also use versions of E&M signaling. E&M is considered an obsolete technology for new installations, which generally use Basic Rate (BRI) or Primary Rate (PRI) digital interfaces. == Signaling units and trunk circuits == The E&M standards were initially developed by Bell Labs and extended by national PTT administrations. The standard defines two sides to the interface: * Trunk Circuit – this is normally the PABX * Signaling Unit – this used to refer to the special modem that converted the DC signaling systems into tones that could be transmitted over a 4-wire link back to the PTT exchange. The Signaling Unit and Trunk Circuit communicate their status over the E and M leads, using a combination of Battery and Earth (also known as Ground). The Battery signal used in the standard is nominally −48VDC. All E&M installations require that the +ve terminal of the Battery is connected to a shared reliable Earth. The maximum distance between the Signaling Unit and the Trunk interface is determined by the resistance of the wire, but will normally be less than 100m for adequate noise immunity. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「E and M signaling」の詳細全文を読む スポンサード リンク
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