Comtech EF Data CRS-170A Installation And Operation Manual page 32

CRS-170A L-Band 1:1 Redundancy Switch
Introduction
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
keeps the Offline modem from interfering with the Rx data traffic. Send timing output from the
Offline modem is also TRI‐STATE
Manual Switching is enabled by the User via the Online modem, through its front panel or by
remote control.
Automatic Switching Selection is controlled by the User via the Online
modem. Then, from the CRS‐170A, the User determines the conditions
that cause an automatic switchover using two DIP switches – Unit+Tx and
Unit+Rx – located on the antenna side of the CRS‐170A and labeled
Switch Conditions. This selection provides a great deal of flexibility in the
operation of the switch: the User can select between Modem Unit faults
only; Modem Unit faults or Receive Traffic faults; Modem Unit faults or
Transmit Traffic faults; or all three.
Also located on the antenna side of the CRS‐170A are two green LEDs that indicate which
modem is Online.
With the 'bridging' architecture of the CRS‐170A (whereby identical terrestrial data traffic
signals are routed to both Online and Offline modems), the redundancy controller can avoid
unnecessary switchovers. By examining the fault status of both modems, it can infer if the fault
is external to the system. For example, the CRS‐170A has been configured to switch following
Unit faults or Transmit Traffic faults, and the modems have been configured for external clock
operation. Now, suppose that the external equipment (network, multiplex, router, etc.) fails –
both the Offline and Online modems will now show a Transmit Traffic fault (No Clock Detected
from Terrestrial Port). The CRS‐170A Controller State Machine will see that both 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.
control of the Offline modem line drivers
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