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SDM-309B Satellite Modem
The transmit data will be replaced with an all ones pattern (Alarm Indication Signal or
AIS) in the event of certain failures, per IESS-309. The modem will transparently pass an
incoming AIS signal but can signal that this is happening if interrogated from the front
panel or remotely through the M&C Interface.
As a test mode the transmit data can be replaced with a 2047 pattern. Selecting this over-
rides AIS. Only user data bits are replaced with the pattern, the Engineering Service
Channel including framing and alarms will operate normally.
The composite multiplexed data stream is normally fed to the modem for further
processing (K7 Viterbi encoding and modulation), but maybe looped back at this point as
a test function called "Interface Loopback". This allows the customer to test virtually the
entire interface as the ESC is looped to itself through the Demultiplexer (Demux). User
data also passes through the a Plesiochronous buffer in this mode, so this may also be
checked. The interface loopback function is bidirectional so that received modem data is
returned to the modulator for transmission. Figure I-2 (Front Panel Menu Tree) shows
the location of the Interface Loopback function.
4.2.1.4 Receive Multiplexer
Receive data including overhead is fed into the demultiplexer for processing (except in
interface loopback). This circuit checks and synchronizes to the frame pattern and
separates the user data from the ESC channel. If the Demux is receiving a correct signal
and synchronized, it will signal the modem that the multiplex system is locked
("Muxlock") and passing data . This is indicated by interrogating the modem and by a
green Light Emitting Diode (LED) on the Interface, and is fed into the receive fault tree
per IESS-309. Under certain fault conditions defined by IESS-309 the receive user data
will be replaced by an all ones pattern. A fault will be signaled when this occurs.
4.2.1.5 Plesiochronous Buffer
User data from the Demux Section is fed into a Plesiochronous buffer. The buffer size is
user selectable in 16 bit increments from 384 to 262144 bits. The buffer is automatically
centered on resumption of service after an outage or may be commanded to center in the
interface configuration Section, from the front panel or remotely. Manual centering will
generally not be Plesiochronous for obvious reasons.
The fill status is available as a monitor function and is accurate to 1%. Overflow or
underflow incidents will be momentarily indicated by red LED's on the module and are
stored in the 'stored Fault" Section of the Monitor & Control status registers, along with
the date and time of the incident, provided by the modem internal clock. These are stored
in battery backed RAM.
The user may select to have the data clocked out of the buffer from 4 clock sources; the
recovered "TX Terrestrial" clock from user terrestrial input data, a user supplied
"External" reference clock, the 'satellite" clock which is the modem receive clock, or the
"Internal" clock source. Satellite clock selection is similar to buffer bypass. Lack of
MN/U-SDM309B Rev. #
M&C and Interfaces
4-4–13
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