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P300H P300 Series Modem Installation and Operating Handbook Page 164
Detected failures
A failure is defined as either a detected failure of the unit, or a catastrophic (power) failure. The Modems
will changeover if one fails, and if the other unit indicates that it is still operational. If both units fail
simultaneously (which is likely to be caused by external circumstances), then no changeover takes place.
Both unit and traffic faults can cause a 1:1 switch over. However traffic faults caused externally to the
equipment which would affect both units simultaneously do not force a spurious changeover, due to the
design of the 1:1 switching logic.
Manual changeovers
The Modem is able to `give away' operation to the other unit of the pair, if instructed to do so from the front
panel, as described earlier. The switch over will only occur if the other unit indicates that it is OK, and is
performed by momentarily simulating a failure in the on-line equipment.
9.3 PRACTICAL 1 FOR 1 IMPLEMENTATION
To implement 1 FOR 1 switching between any two Modem's, the following steps need to be performed:
1. Check that both Modem's have the same software issue.
2. Connect a 4 way screened cable, between the 9 pin 1 FOR 1 connectors on the two Modem's to
be used, refer to Appendix B for details of the connector, as well as the lead required.
3. If the modem has the G.703 option fitted, ensure the switches on the G.703 options are set for 1:1
operation.
4. Plug in the 9 pin 1:1 control lead first, this should force one of the Modem's to indicate that its
outputs are tri-state (the standby LED on the front and the Hi-Z LED on the rear should illuminate
on one modem).
5. Connect the data interfaces in parallel, and configure the two units alike. Suitable adapter cables
are available from Paradise for all the standard interfaces.
6. Connect the two transmit IF ports to the input ports of a suitable power splitter/combiner, of the
correct impedance (50
or 75 ) and the appropriate frequency range. The combined output is
then fed to the up-conversion equipment. Note that only one output is active at a time, and because
of the loss of most splitter/combiners the power level at the output of each modem needs to be
increased by approximately 3.5dB.
7. Connect the two receive IF ports to the two output ports of a suitable power splitter/combiner, of
the correct impedance (50
or 75 ) and the appropriate frequency range. The output of the down-
conversion equipment then feeds the input of the splitter/combiner, so that both demodulators are
fed with an identical signal. Because of the wide dynamic range of the Modem AGC circuits, there
should be no need to modify the signal levels.
8. Switch between units by using the menu option Main, Actions, Giveaway in 1:1. The pair will not
switch over unless the partner unit is operational (ie indicates no faults).
Note: Testing a 1:1 pair in loopback requires the IF's to be split/combined and then looped. Placing an
external individual loopback on each modem will not work as the off-line one who's output is muted
sees a Rx fault as there is no carrier. When split/combined and looped testing will work, but it can
still be difficult to `get your head round it`. It is much easier with no loopback and the 1:1 pair talking
to a third modem.