Comtech EF Data CRS-100 - Manual (Page 27)

Rev. 1.2 4–1
Chapter 4. FUNCTIONAL
DESCRIPTION
The CRS-100 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 and IF signals to and from
the two units. In the case of an equipment failure, switching automatically takes place to
protect the traffic circuit. At the heart of the CRS-100 is a Controller State Machine,
which is responsible for fault monitoring and control of switching functions. It is
implemented in a CPLD.
The unit derives its operating power from the two modems - Online and Standby. A diode
sharing arrangement, with a current sharing circuit ensures that in normal operation
power is taken equally from the two modems. However, in the event that one of the two
units is removed, the remaining modem can supply all of the current requirement. The
Modems supply +12 volts DC (at a combined total of 160 mA max) and -12 volts DC (at
a combined total of 100 mA max). Maximum power consumption occurs in either a V.35
or EIA-422 interface mode. Power consumption in EIA-232 modes is approximately
40% of the maximum values. The modem employ electronic fuses, which prevent
excessive current from being drawn by the CRS-100, should an anomalous condition
occur.
Transmit, Clock, and Data signals entering the unit via the Data Interface are buffered
and fed to both units simultaneously. This 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, using 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-100 provides further
isolation (and security) by using an RF relay within the unit. Unlike some other 1:1
redundancy systems, which use a passive power combiner on the two Tx IF ports (and