Vision Series Installation and Operating Handbook
8-4
8.2.2 Sw itching Philosophy
Both modems operate continuously, w ith each internally monitoring its operational status on a
regular basis. There is no pre-assigned master and slave unit. Sw itchovers betw een units are
minimised and a unit that is taken off-line due to a failure w ill remain off-line even if it returns to
its normal w orking state. If it does return to a nor mal state then it w ill act as the standby unit.
The modem softw are performs internal monitoring of operational status at a rate that prevents
unnecessary switching. How ever, in the case of catastrophic failures, the modem hardw are
performs the sw itch immediately w ithout w aiting for the software. The logic has no memory, i.e. a
unit taken off-line because of a failure w ill operate normally once it becomes operational again,
with the proviso that no unnecessary switchovers are made betw een units.
8.2.3 Detected Failures
A failure is defined as either a failure detected by a unit or a catastrophic power failure. In these
circumstances, the modems w ill sw itch over if one unit remains operational. If both units fail
simultaneously (through external circumstances) then no sw itch over takes place. Both unit and
traffic faults can cause a switch over. Traffic faults that are external to the equipment and that
affect both units simultaneously do not result in any sw itch over.
8.2.4 Manual Sw itchover
The modem user interfaces support manual sw itching of control from the operational unit to the
standby unit, assuming the standby unit is working normally. This is achieved by momentarily
simulating a failure in the operational unit.
8.2.5 1-for-1 Setup Procedure
To set 1-for-1 operation betw een tw o modems, the follow ing steps need to be performed:
1. Check that both modems have the same software issue.
2. Connect a 4-w ay screened cable betw een the 9-pin 1-for1 connectors on the tw o modems
- this forces one modem to adopt the 1:1 standby mode.
3. Connect the data interfaces in parallel and configure the tw o units alike. Suitable adapter
cables are available from Paradise Datacom for all of the standard interfaces.
4. If the Ethernet traffic port is used on the modem, then the tw o traffic ports may be
connected to a hub or other multi-port LAN device using RJ45 crossover cables.
5. Connect the tw o transmit IF ports to the input ports of a suitable pow er splitter/combiner of
the correct impedance (50Ohm or 75Ohm) and the appropriate frequency range. The
combined output is fed to the up-conversion equipment. Note that only one output is
active at a time. Because of the signal loss associated w ith splitter/combiners, the pow er
level at the output of each modem needs to be increased by approximately 3.5dB.
6. Connect the tw o receive IF ports to the tw o output ports of a suitable
pow ersplitter/combiner of the correct impedance (50Ohm or 75Ohm) and the appropriate
frequency range. Both demodulators w ill receive an identical signal from the dow n-
conversion equipment via the splitter/combiner. Because of the w ide dynamic range of the
modem AGC circuitry, there should be no need to modify signal levels.
7. Check correct operation by performing a manual sw itch betw een the units (via the
Unit-
Advanced-Operation
menu). The pair w ill not sw itch over unless the standby unit is
operational (i.e. does not indicate any fault). (Note that the pair can be tested in loopback
mode but this requires the IF signals to be split and combined and looped back to the
