Table of Contents
Advertisement
CRS-180 70/140 MHz IF 1:1 Redundancy Switch
Introduction
•
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 switch 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 switch 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
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‐180, 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‐180 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‐180 are two green LEDs that indicate which modem
is Online.
With the 'bridging' architecture of the CRS‐180 (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‐180 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‐180 Controller State Machine will see that both faults have
1
TRI‐STATE is a registered trademark of National Semiconductor.
®
control of the Offline modem line drivers
®1
.
1–4
Revision 8
MN/CRS180.IOM
Advertisement
Table of Contents
