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DMD50 Universal Satellite Modem
6.2.2 Alarm Masks
The modem performs a high degree of self-monitoring and fault isolation. The alarms for these
faults are separated into the following three categories:
Active Alarms
Common Equipment Alarms
Backward Alarms
A feature exists that allows the user to 'Mask' out certain alarms as explained below.
Masking alarms may cause undesirable modem performance.
CAUTION
When an alarm is masked, the Front Panel LEDs and the Fault Relays do not get asserted, but the
Alarm will still be displayed. This feature is very helpful during debugging or to lock out a
failure of which the user is already aware.
6.2.2.1
Active Alarms
6.2.2.1.1
Major Alarms
Major Alarms indicate a modem hardware failure. Major Alarms may flash briefly during
modem configuration changes and during power-up but should not stay illuminated. Alarms are
grouped into Transmit and Receive Alarms - Transmit and Receive are completely independent.
6.2.2.1.2
Minor Alarms
Minor Alarms indicate that a problem may persist outside the modem such as loss of Terrestrial
Clock, loss of terrestrial data activity, or a detected transmit or receive AIS condition.
Alarms are grouped into Transmit and Receive Alarms - Transmit and Receive are completely
independent.
6.2.2.1.3
Common Equipment Faults
Common equipment faults indicate hardware or configuration problems in the modem that effect
both transmit and receive operation. Most common faults indicate a hardware failure within the
modem, such as a bad power supply. Common faults for the External Reference and External
Clock indicate a bad modem configuration, not a hardware failure.
6.2.2.2
Latched Alarms
Latched Alarms are used to catch intermittent failures. If a fault occurs, the fault indication will
be latched even if the alarm goes away. After the modem is configured and running, it is
recommended that the Latched Alarms be cleared as a final step.
6.2.2.3
Backward Alarms
Backward Alarms are alarms that are fed back to or received from the other end of the satellite
link. In IBS Mode (including Drop & Insert), Backward Alarm 1 is the only one used. It would
be received if the distant end demod drops lock.
MN-DMD50 Revision 4
Maintenance and Troubleshooting
6–4
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