Comtech EF Data CRS-200 - Manual (Page 25)

CRS-200 Redundancy Switch Functional Description
Rev. 1 3–5
switch requires this to operate. Both modem and switch alarms also cause an update to
the switch’s own relay outputs on the rear panel of the System Controller card.
Third, one of the 11 modems is checked for a configuration change on each one second
interval. So, over an 11 second period, any change made to a modem either remotely or
via the front panel will automatically cause an update to that modem’s configuration
which is stored in the switch’s memory. If the switch has been in operation with a modem
or its TMI is missing, so that it has never saved a configuration for that position, that
position is not allowed to become activated. The switch, however, continues to check the
unoccupied position at each pass, so that adding a modem later will result in a
configuration being found by the switch and the modem position may then be activated.
If a modem is removed or fails such that communication is lost, the last stored
configuration remains in the switch’s memory.
After each one second status check, the switch uses the current status information in
conjunction with its own configuration to decide to “backup” or “bridge”. When the
redundant modem “bridges” a particular traffic modem, the redundant modem is re-
configured to match the selected traffic modem using the stored configuration, and is fed
with a copy of the traffic modems signals. To “back-up” that traffic modem, a series of
relay changes on the selected TMI puts the redundant modem online in place of the
traffic modem. In both cases, the RM and the selected TM are in parallel with each other,
meaning that transmit terrestrial signals are sent to both modems simultaneously. By
virtue of being set to the same receive parameters (such as IF frequency and data rate)
and being cabled together, their demodulators will lock to the same signal. Figure 4
shows how the data paths change when a traffic modem is taken offline in place of the
redundant modem. When a configuration change occurs to either the bridged TM or the
RM itself, the switch will automatically reconfigure the RM to match the TM it is
bridging.