7705 SAR Interfaces
176
Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
A PCM SCADA bridge is created using the config>scada bridge-id command and a
branch is created using the config>scada>branch branch-id command.
3.6.3 Redundant Masters
The MDDB and PCM multidrop bridge applications support redundant masters,
where both masters listen to all traffic that is being transmitted from the slaves but
only the active master broadcasts data to the slaves.
There are two modes for master redundancy:
• manual (default mode)
In manual mode, if a master branch fails, the second master branch must be
made active manually with the force-active command in order to receive data
from the master input. The bridge always broadcasts to both master branches.
•auto
In auto mode, both master branch inputs are received simultaneously. This
requires the master input behavior to be similar to an RTU; that is, only the active
master transmits data and the standby master transmits either all 1s (MDDB) or
no data (PCM). If the bridge is in auto mode, the force-active command cannot
be used.
3.6.4 Squelch Functionality
A condition may occur where a single slave continues to send data to the master
after the normal response period has expired. This condition locks up the bridge so
that no other slave can transmit data back to the master. To resolve this condition,
the squelch command can be enabled on a bridge or on an individual slave or
master branch. Squelch is enabled by configuring a timeout period that, once
expired, raises an alarm and triggers the squelching function. A normal quiescent
traffic pattern (all 1s for MDDB and low volume for PCM multidrop) is inserted
towards the bridge. This blocks the problematic slave so that other slaves can
continue to use the bridge.
Note: Larger bridges can be built by cascading individual bridges internally within a single
Integrated Services card and using the master output from one bridge as the slave input to
another bridge. Larger bridges can be cascaded across multiple Integrated Services cards
by using an E&M link.
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