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P300H P300 Series Modem Installation and Operating Handbook Page 163
9 1 FOR 1 OPERATION
9.1 THEORY
Any two Modem's will operate as a 1 FOR 1 redundant pair, with no other equipment except for a single
interconnecting lead, two power splitter/combiners and cable forms to parallel up the interfaces of both
units.
Both Modem's of the pair operate continuously, performing their function on the incoming data and IF
signals, but only one Modem enables its output, and drives the signals towards the terrestrial device and
towards the satellite. For P310 L-Band modems, by default the standby modem also disables and DC &
10MHz reference it might be feeding to a BUC/LNB `up the coax` (this can be changed, See “Relay Mode
4” in Appendix E: Customer Specific Features on page 201 for a description)
The diagram on the right illustrates
the equivalence of a standard
Modem and a 1 FOR 1 redundant
pair. The pair of units, with suitable
cables, may be used as a direct
replacement for a single unit, with
all connectors and pin assignments
remaining the same. A particular
advantage of this arrangement is
that one unit of the pair may be
replaced without interrupting traffic.
To enable the parallelling of all the
interfaces the off line unit provides
no terminating load so the correct
impedance is maintained even
when two are connected in parallel.
All interfaces also have the facility
to disable the outputs (either by
making the outputs high
impedance, or by relay switching),
enabling one of a parallelled pair to
drive the outputs without line
contention.
In the event of a catastrophic failure, such as a power supply etc, the switching circuitry (which is distributed
between the two units) will switch over to the operational unit. All interface cards guarantee not to increase
the load on signal lines in the powered down condition, i.e. the pair will work (and switch over) successfully
even if one unit fails totally.
9.2 SWITCHING PHILOSOPHY
Both Modem's operate continuously, with each providing a regularly updated status to their half of the 1
FOR 1 control logic. There is no pre-assigned master and slave unit.
Changeovers are minimised, i.e. a unit taken off line due to a reported failure, will remain off line even if it
returns to the OK state. If it does return to the OK state then it will act as the standby unit. The software
provides the necessary delays of status to the control logic, to prevent unnecessary switching. There is no
delay in the case of catastrophic failures, when the hardware performs the switching regardless of the
software, and its associated built in delays. The 1 FOR 1 system has no `memory', i.e. a unit taken off line
because of an indicated failure, will be put on line again if it now indicates OK, (but only if the on-line unit
has now failed, otherwise it would be an unnecessary changeover).