E & M SIGNALING

In telephony , an arrangement that uses separate leads, called the "E" lead and "M" lead, for signaling and supervisory purposes. Note 1: The near end signals the far end by applying -48 vdc to the "M" lead, which results in a ground being applied to the far end's "E" lead. When -48 vdc is applied to the far end "M" lead, the near-end "E" lead is grounded. Note 2: The "E" originally stood for "ear," i.e. , when the near-end "E" lead was grounded, the far end was calling and "wanted your ear." The "M" originally stood for "mouth," because when the near-end wanted to call ( i.e. , speak to) the far end, -48 vdc was applied to that lead.