Comtech EF Data SDM-300A - Manual (Page 55)

SDM-300A Satellite Modem Revision 6
Functional Description MN/SDM300A.IOM
2–15
2.6 Backward Alarm Theory and Connections
Four sets of TX and RX backward alarms are available to implement the structure defined in
IESS-308. Backward alarms are sent to the distant side of an IDR link to signal that trouble has
occurred at the receive side (which may have resulted from an improper transmission).
INTELSAT specifies that any major failure of the downlink chain will generate a backward alarm.
Implementation is straightforward in a simple, single-destination link. The modem has a
demodulator fault relay which de-energizes in the event of a receive fault. For this relay to be
connected to the appropriate backward alarm input, the Fault menu for this signal includes the
appropriate overhead framing faults.
This signal also includes faults in the downlink chain, since major problems with the antenna, Low
Noise Amplifier (LNA), down converter, and other components will cause an interruption in
service and fault the modem.
The outputs of the demodulator fault relay are available as follows:
• Pins DF-C and DF-NO on the 50-pin D IDR interface connector (J8)
• 9-pin D modem fault connector (J7)
The relay contacts are named for the faulted state. If a receive fault does not occur, DF-C is
connected to DF-NO. The preferred method of using a backward alarm in a single-destination
system is to connect the signals at the IDR data interface (at the modem in a non-redundant
system, or at the protection switch, if used).
Signal Pin #
GND DB50-2
BWI1 DB50-12
BWI2 DB50-13
BWI3 DB50-14
BWI4 DB50-15
DF-C DB50-16
DF-NO DB50-50
This method signals faults on all four alarm channels, and is compatible with a redundancy system.
The method assumes that the same modem handles traffic in both directions in each
single-destination link.
To handle more complicated systems, connect the appropriate outputs of the Demod fault relay to
the appropriate backward alarm inputs. In a multi-destination system, connect the relay to the
particular backward alarm assigned to that link. Refer to IESS-308 for further clarification.