E and M Signaling - Variants

Variants

There are six main variants of E&M.

• Type I is the most common standard in North America and Japan, and signals an outgoing call to the signaling unit with Battery on the M ("Mouth") lead. Incoming calls are signaled by an Earth (ground) signal on the E ("Ear") lead. The interface is vulnerable to poor earthing at either end, and interference from external electrical noise
• Types II to IV are variants which attempt to overcome the main limitation of Type I, which is the reference to Earth at each end of the circuit. Types II-IV use the Signal Battery (SB) and Signal Ground (SG) lead in conjunction with the E&M wires. This improves noise immunity as the Signal Ground does not carry the same heavy currents as normal Ground connections and provides a low resistance return path for signaling. However, if the main Ground connection fails all Earth current may flow via the interface, causing signaling failure, hum and in extreme cases destruction of equipment.
• Type V is the most common variant in use outside United States. In contrast to Type I, both ends of the connection indicate a call by grounding the relevant lead. This means that it is easy to interconnect two PABXs "back-to-back" by crossing over the E&M leads and transmit and receive pairs.
• SSDC5 is commonly used in the United Kingdom and unlike Type V the on- and off-hook state, are backwards to allow for fail-safe operation. If line breaks interface defaults to busy.