Installation
PSM-500/500L/500LT SCPC Satellite Modem
Page 2-10
PSM-500/500L/500L T- Rev. 0.91
Note: The two modems should be at the same firmware revision for proper redundant operation.
The two modems operate in a “non-priority” redundancy mode, that is, no modem is specified as
“primary”, or having preference when both modems are operational. The first modem turned on
assumes a non-redundancy mode until the second connected unit is powered up. The on-line
unit can be set to send its configuration information to a second unit via the front panel. The
modems will remain in this state, constantly sending status information back and forth until one
unit indicates a failure. If that modem is currently on line, it is switched off-line and the alternate
unit is switch on.
The modem is also capable of operating in 1:N and M:N redundancy switching schemes. The
necessary connections to monitor and control switching are available on the data connector itself
in the form of the modulator and demodulator fault outputs and the auxiliary RS-232 control port.
The alarm outputs are also available. The other facility provided to aid in these redundancy
schemes is the ability to save and recall configuration information. Thus a back-up modem can
obtain and save the configurations from 8 other modems and switch immediately to the
necessary parameters to replace any of those units by simply recalling that unit’s stored
configuration. In addition, the programmable interface and common physical data connector
allows different interface protocols between the primary modems.
A diagram of the connections required for installing 1:1 redundancy is shown in the figure below.
Modem A
Modem B
Aux Xmt
Aux Rcv
Aux Xmt
Aux Rcv
Transmit
IF
Combiner
Receive IF
Splitter
Data "Y" Cable
Paired Modems
Station IF
Equipment
Xmt IF
Xmt IF
Rcv IF
Rcv IF
Figure 2-2 - Modem Connections for 1:1 Redundancy
It is important in L-Band systems to use special splitters and combiners that have the ability to
pass DC used to power the BUC and LNB with sufficient current capacity. One type of these is
termed “Wilkinson” combiners. For the lower power receive LNB connection there are low cost
DC pass combiners that may be suitable. Visit our web site for recommendations.