SLM-3650 Satellite Modem
Revision 3
Options
MN/SLM3650.IOM
A–14
The operator may select from four other clock sources as a backup:
•
Insert clock (D&I mode only)
•
RX satellite
•
Internal clock source
•
TX terrestrial
Problems occurring on any of the selected clocks will cause the modem to substitute the
satellite clock and a fault will be signaled.
A.1.3.2.5
Engineering Service Channel (ESC)
The ESC uses certain bits of the satellite overhead to implement an EIA-232 data
channel. The two types of available data channels are asynchronous and synchronous.
The asynchronous channel works by over-sampling input and output EIA-232 data, so
that a clock signal is not required. Data rates up to 1/2000 of the satellite rate may be
used. Synchronous data channels are also allowed at a rate of 1/512 of the data rate of the
modem. The synchronous channel requires use of the TX clock provided by the modem
for operation.
A.1.3.2.6 Backward
Alarm
A backward alarm signal is included in the D&I overhead. The signal is sent to the
distant side of a satellite link to indicate trouble with the receive side, which may be a
result of improper transmission. The M&C computer monitors the receive side of the
link. In the event of trouble, the M&C sends an alarm over the transmit side to the distant
end. This alarm signal indirectly includes faults in the downlink chain, since major
problems with the antenna, Low Noise Amplifier (LNA), or down converter, etc., will
cause an interruption in service and fault the modem. Reception of a backward alarm is
indicated as one of the events that causes a secondary alarm. The modem may be
interrogated from the front panel or by using the EIA-485 or EIA-232 interface to
identify the cause of the alarm.
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