Comtech EF Data CRS-120 - Manual (Page 18)

1:1 Redundancy Switch Revision 3
Introduction MN/CRS120.IOM
1–4
The operating mode for the 1:1 subsystem is as follows:
Latching mode The online (A) and standby (B) units are established with no faults.
When a failure occurs in the online unit switchover occurs and there
is no further switching unless there is intervention via the remote
control or front panel.
There also are two selections for the Standby Tx-IF, Off and On (DIP switches are located on the
1:1 redundancy switch:
Standby Tx-IF = On This is the default method of operation. With the On selection, the
Standby Tx carrier remains ON provided no Tx or unit fault affecting
the carrier is present.
Standby Tx-IF = Off With this selection, only one modulator 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 because the switch
applies the TX Carrier Off signal to the Standby unit. The CRS-120
provides further isolation (and security) by using an RF relay within
the unit. The RF relay delivers a low loss signal from the IF port of the
modulator to the Online port of the switch compared to the 3.5 dB
loss introduce with methods that use a power combiner.
A special mode of operation is provided for 1:1 redundancy.
Loss Of Tx Data A loss of Tx Data into the modulator will produce a fault into the
modulator. Switchover occurs provided both units do not
simultaneously lose Tx Data. In this mode, the loss of Tx Data does
not turn off the Tx carrier. This alarm is masked under Alarm Mask
section of the menus in the modulator.
Fault status information is fed from each of the two modulators, via the Alarms Interface
connector. Based on the fault status the ‘A’ or ‘B’ is placed online. The 1:1 redundancy switch
simultaneously indicates to the microcontroller within the Standby unit that the unit is no longer
‘Online’. This results in the ‘Online’ LED on the front panel of the unit 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 same time, a green LED will illuminate on the front of the
1:1 redundancy switch to indicate whether the ‘A’ or ‘B’ unit is Online.
The customer selects the operating modes within the modulators and their 1:1 controllers
determine the conditions for switchover. By examining the fault status of both units, it is
determined if the fault is external to the system.