Comtech EF Data CRS-170, Инструкция по установке и эксплуатации

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CRS-170 1:1 Redundancy Switch Revision 1
Installation and Operations Manual MN/CRS170.IOM
24
3.1 O
PERATION WITH
CDM-600L
AND
CRS-150
The CRS-150 connects to two modems, an online unit, and a standby unit, monitors the
fault status of these two units, and controls the routing of data to and from the two units.
IF signals, 10MHz reference signals, DC power for the BUC and LNB, and FSK
signaling to the BUC are switched in the CRS-170 under control of the CRS-150. 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, which 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, via extra power carrying wires in the data cable from each modem.
Similarly, the CRS-170 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 of the current requirement. The modems 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 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 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. Receive 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. In addition, the CRS-170 provides further
isolation (and security) by using an RF relay within the unit.
The Rx IF signal is fed to both modems simultaneously through the power divider in the
CRS-170. This divider does introduce a loss of approximately 3.5 dB, but given the wide
dynamic range of the demodulator in the CDM-600L, 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.