5
3920-A2-GK41-60
February 2002
Model 3921Plus Multiport Carrier-Mounted Modem
The Model 3921Plus multiport is a carrier-mounted modem (Figure 3) equipped with an integral time division
multiplexer (TDM) with a modem sharing device (MSD). The modem installs into the COMSPHERE 3000 Series
Carrier, occupying two slots. The modem’s faceplate covers two slots in the carrier. It has twenty (20) LED status
indicators for displaying modem activity and an audio speaker jack for the carrier’s optional speaker.
The modem’s back panel has four edge card connectors that mount into two connector plates. Each connector
plate has two DB-25-S connectors providing four EIA-232-DTE interfaces.
Both the singleport and multiport Model 3921Plus modems derive ac power from the COMSPHERE 3000 Series
Carrier’s backplane, which is a common bus to all devices installed in the carrier. The user interface to any
3921Plus modem is through the shared diagnostic control panel (SDCP), an optional feature which operates in a
manner similar to the DCP on the 3920Plus modems. For a better understanding of DCP operation, refer to
Chapter 3, DCP Operation, in Document No. 3920-A2-GH30.
49614423a-03
FACEPLATE
EDGE CARD
CONNECTORS
PORT
3
PORT
4
PORT
1
PORT
2
3921
Plus
TXD
RXD
RTS
CTS
DSR
DTR
LSD
Pwr
Alrm
Test
Dial
RI
Busy
Serv
SQ
Status
Front Panel
Spkr
142
125
103
104
105
106
107
108
109
1
3
2
4
Port
BACK
CONNECTOR
PLATES
DTE B
DTE
A
DTE B
DTE
A
Figure 3. Model 3921Plus Multiport Modem
