AT&T MERLIN LEGEND System Reference Manual page 512

Hardware
Merlin II Release 3 Reusable Modules
391A
* While the power supply module can be reused in the system, it does not supply as much power as the 391A1 power
should be replaced
supply module and
protection against
power surges than the 391A1.
5-6 Control Unit
There are different types of lines/trunks for the different functions of each mode.
A Key or Behind Switch system can use the following:
loop-start lines
tie trunks
a DS1 facility programmed for either T1 or ISDN-PRI operation
a ground-start (GS) line only when registered under the MF FCC
classification
A Hybrid/PBX system can use the following:
loop-start trunks
ground-start trunks
tie trunks
direct inward dialing (DID) trunks
a DS1 facility programmed for either T1 or ISDN-PRI operation
A system with a basic carrier has five slots for these modules. Up to two
expansion carriers can be added, each one adding six slots.
The names of modules consist of a number that identifies the number of
line/trunks and/or stations that can be connected to the module, followed by
the types of lines and/or telephones it supports. For example, the 408 GS/LS
module provides four line jacks and eight station jacks and supports ground-
start or loop-start lines.
The system supports 13 types of line/trunk and station modules (see Figure 5-5).
Table 5-1 lists the specifications for each type of module.
The following modules used in a Merlin II Release 3 system can be used in this
system:
391A power supply module*
800 line/trunk module
400 line/trunk module
400 E&M line/trunk module
012 basic telephone module
008 analog station module
408 analog line/trunk and station module
added to the station side of the system. The 391A also has less
if anything is