Comtech EF Data CRS-170A Installation And Operation Manual page 41

CRS-170A L-Band 1:1 Redundancy Switch
Introduction
At the heart of the CRS‐150 is a Controller State Machine that is responsible for fault monitoring
and control of switching functions – it is implemented in a CPLD.
The CRS‐150 redundancy switch derives its operating power from the two modems (online and
standby units) via extra power‐carrying wires in the data cable from each modem. Similarly, the
CRS‐170A derives its +12V power from both modems through 'Y' cable connection to the same
cables. The required online and offline control signals are also included in these cables. A diode
sharing arrangement with a current sharing circuit ensures that power is taken equally from the
two modems in normal operation.
In the event that one of the two units is removed, however, the remaining modem can supply all
current requirements. The modems supply +12 volts DC (at a combined total of 400 mA max)
and ‐12 volts DC (at a combined total of 120 mA max). Maximum power consumption occurs in a
serial LVDS mode at maximum data rate (20 Mbps). Power consumption in RS‐232 modes is
approximately 40% of the maximum values. The modem employs electronic fuses which prevent
excessive current from being drawn by the CRS‐150 should an anomalous condition occur.
Transmit Clock and Data signals entering the CRS‐150 via the Data Interface are buffered and fed
to both modems simultaneously; this parallel feed ensures that the standby unit sees the same
traffic conditions as the online unit. Rx Data and Clock signals coming from the online modem
are routed through signal relays to the Data Interface. In the event that a switchover occurs,
these relays switch so the standby unit then supplies the Data and Clock signals.
As only one modem in the pair (the online unit) is permitted to transmit its IF carrier signal at
any one instant, the standby unit is forced to disable its Tx carrier by asserting the Tx Carrier Off
signal at the Data Interface. Additionally, the CRS‐170A provides further isolation and security by
using an RF relay within the CRS‐170A.
The Rx IF signal is fed to both modems simultaneously through the power divider in the
CRS‐170A. This divider introduces a loss of approximately 3.5 dB but, given the wide dynamic
range of the demodulator in the CDM‐600L (CLM‐9600L), this is not considered to be a problem.
The advantage of this scheme is that the demodulators in both online and standby units are
locked, and therefore if a switchover does occur, there will be no delay while waiting for the
demodulator to acquire lock.
Fault status information is fed from each of the two modems via the Data Interface connector.
Based on the fault status, the Controller State Machine in the CRS‐150 decides whether the 'A'
or 'B' unit is to be the online unit. It asserts a control signal to the standby unit, which mutes its
Tx IF carrier and simultaneously indicates to the microcontroller within the standby unit that the
unit is no longer 'online'. Consequently, the 'online' LED on the front panel of the standby
modem is extinguished and the standby status is also reported over the remote control bus so
an external M&C system can determine the state of the redundancy system. At the same time, a
green LED will illuminate on the front of the CRS‐150 to indicate whether the 'A' or 'B' unit is
online, and a green LED on the antenna side of the CRS‐170A will indicate the same status.
DIP switches on the CRS‐170A's front panel are set to establish modem‐specific operation. With
the CDM‐600L (CLM‐9600L), place the Mode switch in the SDM‐300L/CDM‐600L (CLM‐9600L)
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MN/CRS170A.IOM