Quantum and Evolution Series Installation and Operating Handbook
8-20
The pair of units, connected as shown, 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 parallel operation of all the interfaces, the off-line unit provides no
terminating load, enabling the correct impedance to be maintained even when the two
modems are connected in parallel.
All interfaces have the facility to disable their outputs (either by making the outputs high
impedance or by relay switching) thereby enabling one of a redundant pair to drive the
outputs without line contention.
In the event of a catastrophic failure of one modem, the switching circuitry (which is
distributed between the two units) will switch over to the operational unit.
8.4.2 Switching
Philosophy
Both modems operate continuously, with each internally monitoring its operational status
on a regular basis. There is no pre-assigned master and slave unit. Switchovers between
units are minimised and a unit that is taken off-line due to a failure will remain off-line
even if it returns to its normal working state. If it does return to a normal state then it will
act as the standby unit.
The modem software performs internal monitoring of operational status at a rate that
prevents unnecessary switching. However, in the case of catastrophic failures, the
modem hardware performs the switch immediately without waiting for the software. The
logic has no memory, i.e. a unit taken off-line because of a failure will operate normally
once it becomes operational again, with the proviso that no unnecessary switchovers are
made between units.
8.4.3 Detected
Failures
A failure is defined as either a failure detected by a unit or a catastrophic power failure. In
these circumstances, the modems will switch over if one unit remains operational. If both
units fail simultaneously (through external circumstances) then no switch 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 switch
over.
8.4.4 Manual
Switchover
The modem user interfaces support manual switching 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.
8.4.5 1-for-1 Setup Procedure
To set 1-for-1 operation between 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-for1 connectors on the two
modems - this forces one modem to adopt the 1:1 standby mode.
