Comtech EF Data CRS-170A - Manual (Page 37)

CRS-170A L-Band 1:1 Redundancy Switch MN/CRS170A.IOM
Introduction Revision 13
1–17
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 EIA-232 modes is
approximately 40% of the maximum values. The modem employs electronic fuses that 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 Backup unit sees the same
traffic conditions as the Online unit. Rx Data and Clock signals coming from the Online unit are
routed through signal relays to the Data Interface. In the event that a switchover occurs, these
relays switch so the Backup 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 Backup 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 unit.
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 Backup 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 the Online and Backup 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 Backup unit,
which mutes its Tx IF carrier and simultaneously indicates to the microcontroller within the
Backup unit that the unit is no longer ‘Online’. Consequently, the ‘Online’ LED on the front panel
of the Backup Modem is extinguished and the Backup 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 the green LEDs on the antenna side of the CRS-170A will indicate the
same status.
Set the DIP switches on the CRS-170A antenna side to establish
modem-specific operation. For the CDM-600L (CLM-9600L), place
the Mode switch in the SDM-300L/CDM-600L (Dumb Controller)
position and push the Switch Condition switches to the OFF
position.
Sect. 4.9 Configure Switch DIP Settings