P300H P300 Series Modem Installation and Operating Handbook Page 167
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 Modems’, the following steps need to be performed:
1. Check that both Modems’ 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
Ó¿·²ô ß½¬·±²-ô Ù·ª»¿©¿§ ·² ïæï
. 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.
