Backward Alarm Theory And Connections - Comtech EF Data SDM-300A Installation And Operation Manual

Theory of Operation
4.6

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). 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:
•
•
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).
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.
4–16
Pins DF-C and DF-NO on the 50-pin D IDR interface connector (J8)
9-pin D modem fault connector (J7)
Signal
GND
BWI1
BWI2
BWI3
BWI4
DF-C
DF-NO
Pin #
DB50-2
DB50-12
DB50-13
DB50-14
DB50-15
DB50-16
DB50-50
SDM-300A Satellite Modem
Rev. 4