Comtech EF Data CRS-150, Installation And Operation Manual

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CRS-150 1:1 Redundancy Switch Revision 2
Introduction MN/CRS150.IOM
1–4
1.3 Functional Description
The CRS-150 connects to two modems – the online modem (identified for the purpose of this
example as Unit ‘A’), and the redundant (standby) modem (Unit ‘B’). The CRS-150 monitors the
fault status and controls the routing of data and IF signals to and from these two modems. In the
case of an equipment failure, switching automatically takes place to protect the traffic circuit.
At the heart of the CRS-150 is a Controller State Machine. Implemented in a complex
programmable logic device (CPLD), it is responsible for fault monitoring and control of switching
functions.
The CRS-150 derives its operating power from the Unit ‘A’ and Unit ‘B’ modems. A diode-
sharing arrangement with a current-sharing circuit ensures that, in normal operation, power is
taken equally from the two modems. However, in the event that one of the two units is removed,
the remaining modem can supply all of the current requirement.
The modems 12 volts DC (at a combined total of 250 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 EIA-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 unit via the Data Interface are buffered and fed to
both units simultaneously. This ensures that the standby unit sees the same traffic conditions as
the online unit. Receive Data and Clock signals coming from the online modem are routed, using
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. In addition, the CRS-150 provides further isolation (and security) by
using an RF relay within the unit. Unlike some other 1:1 redundancy systems that use a passive
power combiner on the two Tx IF ports (and hence lose approximately 3.5 dB in output power
level) the CRS-150 does not introduce any significant attenuation of output signal level.
The Receive IF signal is fed to both units simultaneously using an internal power divider. This
does introduce a loss of approximately 3.5 dB but, given the wide dynamic range of the
demodulator in the CDM-600, this is not considered to be a problem. The advantage of this
scheme is that the demodulators in both the online and standby units are locked; therefore, if a
switchover does occur there will be no delay while waiting for the demodulator to acquire lock –
this greatly speeds the time for the switchover to occur.
Fault status information is fed from each of the two modems via the Data Interface connector.
The Controller State Machine decides, based on the fault status, which of the two modems – Unit
‘A’ or Unit ‘B’ – is to be the online modem. It will assert a control signal to the standby modem,
which mutes its Tx IF carrier, and simultaneously indicates to the microcontroller within the
standby unit that the modem is no longer online. This results in the “ON LINE” LED on the now-
offline modem’s front panel being extinguished. This 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