AT&T MERLIN LEGEND Installation & Maintenance Manual page 177

Installation
This allows data communications between digital data stations
and analog data stations.
There are two types of modem pools:
Analog to Digital converts analog signals to digital signals.
This allows analog data station users to communicate with
inside digital station users or to place data calls using an
outside digital ISDN-PRI facility. The modern is connected to
the control unit by a basic station jack on a 012 module, and
the ISDN 7500B Data Module is connected to the control unit
by a digital station jack on a 008 MLX module. Data calls are
placed to outside data stations through the control unit using
system lines (outside ISDN-PRI facilities connected to the
DS1 line/trunk jack on a 100D module in the control unit).
Digital to Analog converts digital signals to analog signals.
This allows digital data station users to communicate with
inside analog data stations or to place data calls using the
regular telephone network.
The modem is connected to the CU by a tip/ring jack on a
012 or 008 OPT module, and the ISDN 7500B Data Module is
connected to the CU by a digital station jack on a 008 MLX
module. Data calls are placed to outside data stations
through the CU using system lines (outside tines/trunks
connected to a line/trunk jack on a 400, 400 GS/LS/TTR 800,
800 GS/LS, 408, 408 GS/LS, 800 DID, or 400EM module in
the CU.
Analog-to-digital and digital-to-analog pools can also connect
dedicated outside analog data lines to digital ports (008 MLX
module) on the system.
Because of interfacing problems between the RS-232 on the
modem with the RS-232 on the 7500B, modem pools can
operate in one direction only—analog-to-digital or digital-to-
analog.
Data Adapters 3-69