Comtech EF Data CRS-200 REDUNDANCY SWITCH - Manual (Page 17)

CRS-200 COMTECH COMMUNICATIONS CORPORATION
CRS-200 REDUNDANCY SWITCH - OPERATIONS MANUAL PAGE 15
A CRS-240 Power Supply Module is installed in each side of the chassis. If one module is
removed while power is on, the system continues to operate with no disturbance. This allows
servicing of a module without interruption. The controller monitors the +5V, +12V and –12V
supplies of each module separately, and any fault reported on either module can be masked
if the user chooses to operate without a backup in place. Module A is on the right and
Module B is on the left as viewed from the rear of the chassis.
The microcontroller in the CRS-200 continuously scans the system to gather information. This
information is then used to determine if action needs to be taken. Every second, three status
checks are performed. First, all 11 modem interface positions are checked in succession for
the presence of an interface card and, if present, all three modem alarm relay conditions are
checked. If a modem interface is present but no modem is connected with the 25-pin cable,
all three fault indicator LEDs on the front panel will show the faulted state. Unmasked faults
are logged.
Second, the switch checks itself for faults. These consist of either power supply faults or a
failure to communicate remotely with the redundant modem, and are also logged if
unmasked. The outputs of both power supplies are monitored for both overvoltage and
undervoltage conditions, but only one supply may have its faults masked at a time. A remote
communication fault with the redundant modem may not be masked, since the 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.