User's Manual

455U-D Radio Modem Module User Manual
Page 28 © Jan 2008
2.6 Dual Redundant Installation
Two 455U-D units can be connected together in a dual redundant arrangement. One unit
configured as a Primary is normally active, and the Secondary unit is inactive. If the Primary
detects an internal fault or operating problem, then the Primary unit becomes inactive and the
Secondary becomes active.
The trigger conditions for a change-over are:
♦ Complete failure of the active 455U - power failure, CPU failure;
♦ Loss of radio communications for a pre-configured time - radio “comms-fail”;
♦ Loss of serial communications for a pre-configured time - serial “comms-fail”;
♦ Fault detected by internal diagnostics - over-temperature, antenna fault (high SWR),
frequency failed-to-lock, or transmitter PTT timeout;
♦ Operator request via configuration.
The 455U units will only change-over if the other unit is clear of faults. To prevent change-
over during short term transient fault conditions, the trigger condition must be present for a
time period before the change-over occurs (“change-over delay time”, user-configurable). If
the primary unit changes over to the secondary unit, the secondary unit remains as the active
unit until it detects a fault, or until it is forced to change by manual configuration.
The inactive unit continues to operate, and can receive messages via the radio and data ports.
However it is disabled from transmitting messages on the radio and RS485 ports - it can
transmit messages on the RS232 port.
Single Host Device
If the 455U redundant pair are connected to a single host device, they should be connected via
RS485 - both 455U units will hear all data messages, but only the active 455U unit will send
RS232
RS232
RS485
REDUNDANT PAIR WITH SINGLE HOST REDUNDANT PAIR WITH REDUNDANT HOSTS
USING RS485 USING RS232