Comtech EF Data SDM-300A - Manual (Page 352)

SDM-300A Satellite Modem Revision 6
Troubleshooting MN/SDM300A.IOM
18–16
18.3.3.4 Receive Interface Faults
Fault/Alarm Possible Problem and Action
BUFFER
UNDERFLOW
Buffer underflow alarm.
Indicates the plesiochronous buffer has underflowed. Buffer underflow is normally a momentary fault
(there are clock problems if this alarm is continuously present). This alarm is included in this section to
be consistent with the fault reporting system and to be correctly registered in the stored fault memory.
The time and date of the first 10 receive buffer underflow faults are stored in battery-backed memory
as an aid to troubleshooting. The interval between stored overflow/underflow events can be used to
determine relative clock accuracies.
BUFFER
OVERFLOW
Buffer overflow alarm.
Indicates the plesiochronous buffer has overflowed. The problems and actions in the buffer underflow
section apply to this alarm.
RX DATA/AIS Data or incoming AIS. The data monitored for RX data is coming from the satellite.
When the AIS is selected for RX data fault in the Interface Utility menu, the RX data/AIS is monitoring
an alarm condition of all 1s from the satellite. When data is selected for RX data fault in the Interface
Utility menu, the RX data/AIS is monitoring a fault condition of all 1s or 0s. This is referred to as a
data-stable condition (data is not transitioning). The fault indicates trouble in receiving data from the
satellite. The modem passes this signal transparently, and can close a FORM C contact. The
indication is a monitor function only to help isolate the source of trouble in a system.
FRAME BER The receive decoded error rate has exceeded 10-3 over a 60-second period measured on the framing
bits.
This is defined as a major (prompt) receive alarm by INTELSAT specifications IESS-308. In a
redundant system, a switch-over will be attempted. Since some data must be correctly received to
indicate this fault, receive AIS will not be substituted. This fault is to be sent as a backward alarm to
the distant end. This must be wired externally, as faults other than from the modem may need to enter
the fault tree.
BACKWARD
ALARM
Backward alarm. Modem is receiving a backward alarm, indicating trouble at the distant end. The
alarm may be a result of improper transmission at the near end of the link.
This particular alarm is reported and recorded, but the modem takes no other action. In most cases,
the fault is due to some receive problem with the modem, so a real fault will probably be occurring if
backward alarm faults are being recorded.
BUFFER CLK PLL Buffer clock phase-locked loop fault. The buffer synthesizer is the wrong frequency or will not lock.
Ensure the selected buffer clock source is at the proper frequency and level. If the fault continues,
contact the Comtech EF Data Customer Support Department.
BUFFER CLK ACT Activity detector alarm of the selected interface receive clock.
The interface will fall back to the satellite clock when this fault is active.