Thermo Scientific AutoWAVE 1-0479-013, Руководство пользователя

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Document 1-0479-013 Rev B
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5.0 MULTIPOINT OPERATION
In a Multipoint system a transceiver 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. As such, 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 smaller the system capacity.
2. RS232 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.
Setting Multipoint Parameters
(0) Number Repeaters
In a Multipoint network it is critical for timing purposes to know whether or not there are
repeaters in the network. Any transceiver that is used as a repeater essentially becomes a
master to the slaves and other repeaters to which it is communicating. Therefore, the user
must identify whether or not the network contains repeaters. This is done by assigning a
value in parameter (0), Number Repeaters. The value should be 0 if there are no repeaters
in the network and 1 if repeaters are present. This parameter does not need to be set in the
repeaters or slaves in a Multipoint system.
(1) Master Packet Repeat
In point-to-point operation the AutoWAVE transceivers acknowledge every data packet
transmitted. In a Multipoint network, transmissions from a master to the slaves are not
acknowledged by the slaves. This is to prevent system overload. If the slaves acknowledged
all data transmissions from the master in a large Multipoint system, then all system capacity
would be spent having the master listen for acknowledgments from the slaves. Because the
transmission is not acknowledged by the slaves 100% confidence does not exist that every
slave has received every message from the master. To address this issue the user may
modify option (1) Master Packet Repeat, assigning a value between 0 (the packet is
transmitted once) to 9 (the packet is repeated 9 times). For networks with solid RF links,
this parameter would be set at the lower end of the scale (0-1). If the network has some
weak or marginal links it would be set toward the higher values. If a slave receives a packet
from a master more than once it will discard the repeated packets received.