Comtech EF Data CRS-180 Manual page 22

FUNCTIONAL DESCRIPTION
CRS-180 IF 1:1 Redundancy Switch
Manual switchover is enabled from the front panel or remote control of the online CDM-
570 modem. Automatic switching is inhibited by the user from the online unit. The user
can select Unit Faults only, Unit Faults or Receive Traffic Faults, Unit Faults or
Transmit Traffic Faults, or all three for the switchover criteria with the two switches on
top of the unit. This selection provides a great deal of flexibility in the operation of the
switch. Green LEDs on the antenna side of the CRS-180 indicate which modem is online.
The control cable from each CDM-570 modem to the CRS-180 also includes +12V
power to operate the CRS-180. Power from both modems is diode OR'd so that the
switch remains active if power is lost from one of the modems.
The CRS-180 provides IF redundancy.On the Tx side, it switches the Tx IF signal. The
Rx IF signal from the RF equipment 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 at the CDM-570 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
keeps the offline modem from interfering with the Rx data traffic. Send timing output
from the offline modem is also TRI-STATE
With the 'bridging' architecture of the CRS-180 (whereby identical traffic signals are
routed to both Online and Standby units), the Controller State Machine can avoid
unnecessary switchovers. By examining the fault status of both units, it can infer if the
fault is external to the system.
For example, suppose the CRS-180 has been configured (via the Dip Switches) to
switch following Unit faults or Transmit Traffic faults, and that the modems are
configured for external clock operation. Now suppose that the external equipment
(network, mux, router, etc.) fails. Both the standby and the Online units will now
show a Transmit Traffic fault (No Clock Detected from Terrestrial Port). The CRS-
180 Controller State Machine will see that both these faults have occurred at the
same time (in fact, within a 0.5 second window), and infers that the fault is external.
Therefore, no unnecessary switchover is initiated.
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TRI-STATE is a registered trademark of National Semiconductor.
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MN/CRS180.IOM
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