AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 142

Lines and Trunks
E&M mode. This is a standard interface. The E&M signaling leads are
isolated from the transmission leads, requiring a 3-pair wire interface.
In the simplex mode, protective resistance is always included in the circuit. The
E&M mode can be either protected or unprotected from high-voltage transients
or fluctuations. In the protected mode, a resistance is added to the leads to
reduce current peaks, The protected mode is used when there is no network
interface to protect the circuit from outside interference.
The unprotected mode must be used for an E&M Type 1 Standard interface
(see below) to meet the specified voltage-drop criteria. This mode is used when
there is a network interface.
Signaling Types
Three different signaling types combine with the signaling modes. Together
these create the proper signaling format for each system.
Type 1 Standard. This is the factory-set type, which is used to connect
two systems to the network through two intermediate telephone company
COs. The switches must be set for E&M mode.
Type 1 Compatible. This directly connects two systems without
intermediate telephone company COs. One system is set to Type 1
Standard, the other to Type 1 Compatible. The switches must be set for
E&M mode.
Type 5 simplex or E&M. This type is used to connect similar systems or
systems with compatible signaling that are located in the same building
or on the same business campus.
The choice of a tie-trunk signaling format to connect two systems depends on
the particular application and the systems being connected, including whether
or not the tie-trunk signals pass through telephone company lines or over
customer-owned cable. Table 3-3 shows how to determine tie-trunk
compatibility between this system and other systems.
3-8 Tie Trunks