3020-dda.ib.rev0.doc
Page 9 of 17
09/03/2006
Handshake operation:
Purpose:
Handshake interconnection is required when two circuits are to be operated in 1:1 protection switching mode to
provide a continuous signal output in the event of failure of the primary signal path.
Priority logic:
For this mode to be employed it is necessary to provide two programme feeds which are designated as the
Main
and
Standby
paths.
The priority switching in normal mode follows non-reverting logic, which dictates:
In the event of failure of main then standby DDA will assume control and become
Main
causing the failed
path DDA to become
Standby
.
This implies that when the failed path is restored that it will remain as
Standby
and not become
Main
unless either a
failure of
Main
occurs or a manual changeover is requested.
Changeover logic:
A changeover to the companion module will occur under any of the following conditions:
Loss of input signal
AIS detection alarm (provided AIS is not disabled by link LK 3)
Loss of power
In all of the above cases switching will only occur if:
the companion module is able to provide an output free of the same defects AND
the changeover inhibit switch is not activated on either module.
Connections:
Handshake interconnection should only be made using either the ZDA-3021RH handshake double rear assembly.
This rear assembly makes all the necessary connections for both logic and data signals when two DDA-3020’s are
inserted side by side in a 3 RU frame. Individual alarm outputs are provided for each module.
Logic connections:
All required logic connections are made by tracks on the double width PCB. Automatic operation is immediately
initiated when two modules are plugged into this type of rear assembly.
No external connections are required, but external alarm connections are available from each module for use if
desired. Additionally, each module has a connection for an external
Make Main
control for remote DA selection.
Handshake mode detection:
Two data lines are present on the handshake connector to indicate to each module that it is to operate in handshake
mode.
Power on reset.
When power is applied to the pair the
power on reset
signal will attempt to reset both modules. However, as only
one module can be
Main
, the logic processor checks for handshake operation and if detected then the module, which
was last enabled as
Main
will take control as
Main
and the other module will be forced to act as
Reserve
.
This memory capability is due to the latching nature of the RL1 relay, which will cause the
Main
and
Reserve
paths
to be maintained even in the absence of power.
The only exception to this rule is when power is applied to a pair for the first time that they are coupled in
handshake mode. In this special case both modules will initially have their K 3 relays in the active path condition
and so both will attempt to become
Main
. As the P.O.R. signal for each module will be slightly different for any two
modules, one will reach its operating mode first and will force the other module to immediately change to become
Reserve.
As the selection of which module becomes
Main
is cannot be determined before installation it may be necessary to
force the desired module to become main by pressing the
Change Request
button on the front panel of the desired
module. The
Main
module will be indicated by the
In Service
LED, on the front panel, lighting.
Automatic changeover:
An automatic changeover is initiated whenever the power fails on
Main
and not on
Reserve
or when a general alarm
is initiated on
Main
(indicating either loss of input signal or AIS indication if the AIS is enabled) and the
Change
Inhibit
switch is not active on either module.
Manual changeover:
A manual changeover is initiated by pressing the
Change Request
button on the front of the module that is required
to become
Main
.
The mechanism of the change is similar to the automatic changeover described above except that it is initiated by
the module requesting that it become
Main
.
IRT
Communications
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