11
SHM-768K
Figure 2-2. Tail-end for DDS Service Application.
The SHM-768K is coupled to the line through isolation transformers which,
in conjunction with protective circuitry, protect against AC or DC
overvoltages. The protective circuitry enables operation even when DC is
connected to the line.
2.2 Physical Description
The SHM-768K is available as a desk-top unit or as a rack-mount card for a 19"
rack. The rack can carry up to 14 SHM-768K cards and is supplied with 25-pin
D-type connectors. Optional interface attachments (CIA/V.35 or CIA/X.21)
provide two V.35 or X.21 connectors for any two adjacent cards. Up to seven
attachments can be connected side by side.
2.3 Functional Description
This section contains functional descriptions of the SHM-768K circuit blocks.
These descriptions focus on the circuits required for setting the correct
configuration of the modem. Refer to the SHM-768K Block Diagram,
Figure 2-3.
2.3.1 T
RANSMITTER
CDP Encoder
The encoder modulates the input (NRZ) data from the DTE using the
“conditional diphase modulation” technique. The carrier may be ON
constantly or controlled by RTS.
ASM-45
ASM-45
RCV CLK
V.54 DLY-Off
RCV CLK
V.54 DLY-Off
EXT CLK
V.54 DLY-On
EXT CLK
V.54 DLY-On
ASM-45
ASM-45
Digital
Service
Network
SHM-768K
SHM-768K
SHM-768K
RCV CLK V.54
DELAY OFF
SHM-768K
RCV CLK V.54
DELAY OFF