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The operator may select from four other clock sources as a backup:
•
•
•
•
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.
A–14
Insert clock (D&I mode only)
RX satellite
Internal clock source
TX terrestrial
SDM-300A Satellite Modem
Rev. 4
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