Multipoint Operation; Setting Multipoint Parameters - Black Box MDR100A-R4 User Manual

INDUSTRIAL MODEM RF115

6. Multipoint Operation

In a multipoint system, a modem designated as a master is able to simultaneously
be in communication with numerous slaves. In its simplest form, a multipoint
network functions with the master broadcasting its messages to all slaves, and slaves
responding to the master as appropriate.
Traditionally, a multipoint network is used in applications where data is collected
from many instruments and reported back to one central site. The architecture of
such a system is completely different from point-to-point applications. The
theoretical maximum number of slaves that can be configured into a multipoint
network is a function of the data throughput needed from each of the slaves. For
example, if the network will be polling slaves once a day to retrieve sparse data,
several hundred slaves could be configured to a single master. If, on the other
hand, each slave will be transmitting data at greater levels, then fewer slaves may be
connected to the master (the overall system will be closer to capacity with fewer
slaves). The theoretical limit of a multipoint system is influenced by the following
parameters:
1. Size of the blocks of data. The longer the data blocks, the greater the system
capacity.
2. RS-232 baud rate.
3. The amount of contention between slaves.
4. Use of repeaters. A single repeater in a multipoint network will decrease
overall system capacity by 50%; more than one repeater does not further
decrease network capacity.

6.1 Setting Multipoint Parameters

In a multipoint network, it is critical to know how many modems are being used as
repeaters. Any modem that is used as a repeater essentially becomes a master to
the slaves and other repeaters to which it is communicating. Therefore, you must
first identify how many repeaters are connected to the master by assigning a value
in parameter (0) Number Repeaters. This parameter must also be set for each
repeater in the system (that is, if a repeater is connected to one or more other
repeaters). This parameter does not need to be set for Multipoint slaves.
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