3280-dda.ib.doc
page 8 of 16
23/07/2002
Description of Operation
See block diagrams commencing on page 4 and schematic diagrams at rear of this manual.
Input:
The G.703 input signal is connected to the input circuit of the amplifier.
If no power is present, and the amplifier was active before the power was removed, then the signal will be directed
to the output connector via K 1 and K 3. When power is applied the K 1 relays operate and switch the signal via the
amplifier to the K 3 relay and thence the output connectors.
In this power fail mode, the connection from input to output is passive and so only one output can be connected.
The K 3 relay is of the bi-stable magnetic latching type and so will only change state when an imbalance of drive
occurs between the
set
and
reset
coils. The K 3 relay is operated under the control of the logic processor
main select
via transistor driver Q 8 to the
set
coil and released by any external signal present via pin 15a of the rear connector
to the
reset
coil.
When power is applied to the DDA, the logic circuit will cause the K 3 relay to be
set
. If two DDA’s are connected
in handshake configuration, one of the two will take control first and become the
main
amplifier causing the other to
become the
standby
.
Either module can be made
main
by pressing the change request button on the front panel of the required module
provided that no alarms are present. An external changeover request may also be made via pin 3 of SK 2
alarm/control connector on the rear panel provided that the changeover inhibit switch on the front panel is not in the
inhibit position.
When the K 3 relay is in the
reset
position the input path from the input connector and the K 1 relay is terminated in
75
Ω
to preserve the loading on the input.
Active path:
Input switching & equaliser:
The signal from the K 1a relay connects to the input transformer,T2, and data/clock recovery IC, U1, which
genve and –ve mark signals and the received clock.
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Communications
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