Comtech EF Data CRS-180 - Manual (Page 24)

CRS-180 70/140 MHz IF 1:1 Redundancy Switch MN/CRS180.IOM
Introduction Revision 11
1–4
1.3 Functional Description
The CRS-180 performs the Tx and Rx coaxial switching required for redundant modem operation
with an outdoor BUC and LNB.
1.3.1 Switch Operation with CDM-625/A, -760, -750, -570/A, -710G, -710, or
-700 Modems
The CRS-180 is configured with two modems to form a complete 1:1 redundant modem system.
Within the CRS-180, a redundancy controller continually monitors the status of faults for both
modems to determine which modem should be Online and Offline. The control cables between
the modems and the CRS-180 include four signal groups:
• First, serial full-duplex communication lines are routed from one modem to the other
modem via the CRS-180. Using these lines, this Online modem can interrogate the Offline
modem at regular intervals to determine its configuration. If a difference in configuration is
detected, the Online modem will reconfigure the Offline modem so that the configurations
are always synchronized. This process is entirely automatic – if the Offline modem is
replaced, it does not have to be manually reprogrammed to match the Online modem.
• Second, the Modem’s Unit, Tx and Rx faults signals from both modems are routed to the
CRS-180. This allows the redundancy controller to always know the health status of both
modems.
• Third, the Online/Offline signals are routed from the CRS-180 to each modem. Based on the
modem faults, the CRS-180 will determine which modem should be Online and Offline. In
the event that both modems indicate faults through some malfunction, the CRS-180 will
select, by default, Modem ‘A’.
• Fourth, +12V is routed from each modem to the CRS-180 for operational power. This power
from both modems is diode OR’d so that the CRS-180 remains active if power is lost from
one of the modems.
IF Switching – The CRS-180 provides IF redundancy, but when switching occurs, it
accommodates more than just the IF signals. On the Rx side, the Rx IF signal from the LNB is split
in a power divider to continuously drive both the Online and Offline demodulator. The Offline
demodulator can then maintain lock so that it is ready to quickly assume Online status for
receive traffic when a switchover is required.
Data Switching is accomplished inside each modem, in the data interface itself. A ‘Y’ cable
connects both the Online modem and the Offline modem to the terrestrial data source.
Transmit data drives both modems in parallel so that the standby modulator is always ready to
come Online. Termination impedance for the modem line receivers is active in the Online
modem. On the Rx side of the interface, TRI-STATE ® control of the Offline modem line drivers