Comtech EF Data SDM-9000 - Manual (Page 156)

Theory of Operation SDM-9000 Satellite Modem
5–28 Rev. 4
5.6 Backward Alarm Theory and Connections
Four sets of transmit and receive 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).
Implementation is straightforward in a simple, single destination link. INTELSAT
specifies that any major failure of the downlink chain is to generate a backward alarm.
The modem has a receive fault relay which de-energizes in the event of a receive fault. In
order for this relay to be connected to the appropriate backward alarm input, the fault
tree 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 receive fault relay are available as follows:
• Pins DF-C and DF-NO on the 50-pin D 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 modem single destination system is
to connect the signals at the IDR data interface, either 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.