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17 — DNIC and 2B1Q modules
17.10
Multidrop data bridge
In a multidrop data bridge, several broadcast destination circuits (called slaves) take
turns using the same bandwidth to communicate with one broadcast source circuit
(called the master); for example, several terminals would take turns communicating
with a host computer. The master device controls the use of the shared bandwidth by
polling each slave in turn. Figure 17-3 shows a multidrop data bridge.
To create a multidrop bridge, two components are needed: a data interface that
supports the multidrop function and a multidrop data bridge. The 2902 MainStreet
unit supports the multidrop interfaces but does not provide the multidrop data bridge.
One way to use a 2902 MainStreet unit in this type of application is in conjunction
with a 3600 MainStreet Multiservice Bandwidth Manager containing a Sub-Rate
Interface module (SRM).
Master and slave circuits must be configured with the same starting data position and
transport bandwidth (see chapters 21 and 22). To facilitate this, the 2902 MainStreet
system automatically sets the master circuit configuration to full duplex and
signaling off, and the slave circuit configuration to half duplex and signaling off.
These settings allow you to overlap the position of the data from these circuits.
For circuits in a multidrop data bridge, if the circuit is configured as gender DCE, the
DCD control signal must be forced on. If the circuit is configured as gender DTE, the
RTS control signal must be forced on.
Do not use the copy adjust function when configuring circuits that are to be part of
the same multidrop data bridge.
17-12
Figure 17-3 Multidrop data bridge
Responding
Branch
Multidrop
data bridge
circuits
Slave
devices
Polling
Aggregate
circuit
Master
device
2902 MainStreet Technical Practices
90-2906-01
2285
May 2002
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