AT&T MERLIN LEGEND Release 3.1 Installation Manual page 67

Installing the Modules
Tie Trunks
Tie trunks connect two separate PBX communications systems either directly
or through one or more central offices, as if they were one system at the same
location. In the MERLIN LEGEND Communications System, the 400EM
module is the originating and terminating unit for tie trunk operation.
Tie Trunk Signaling
The 400EM (tie trunk) module can transmit signals in three different formats.
Each format is made up of a specific signaling mode and a specific signaling
type. The DIP switches on the 400EM module allow you to select the signaling
mode for tie trunk transmission. The signaling type is selected through system
programming.
Signaling Modes
There are two signaling modes:
E&M. This is a standard interface. The E&M signaling leads are
separate from the transmission leads, requiring a 3-pair wire interface.
Simplex. In Simplex, the two signaling leads are superimposed onto
the analog transmission leads, providing a 2-pair wire interface for
connecting two local systems at minimal cost.
Protected or Unprotected
Depending on the type of tie trunk installation, protective resistance may be
installed to protect the circuit from outside interference from high-voltage
transients or voltage fluctuations. In Simplex mode, the circuit always requires
protective resistance. The E&M mode can be either protected or unprotected,
depending upon the location of the distant PBX.
Signaling Types
Three different signaling types combine with the signaling modes. Together
these create the proper signaling format for each system. Signaling types in
each mode are as follows:
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Installing the Control Unit