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.
Read more about this topic: E And M Signaling
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