Dataradio Viper-1000, Руководство пользователя

Страница: 18 / 113

Viper User
Manual (001-5008-000 Rev6)
Page 18
1.1.10 Point-to-Multipoint
A Point-to-Multipoint network is a common network type used in SCADA or other polling
systems. The single polling master station communicates with any number of remotes and
controls the network by issuing polls and waiting for remote responses. Individual PLC/RTU
remotes manage addressing and respond when their individual addresses are queried.
PLC/RTU unit addresses are maintained in a scanning list stored in the host program or
master terminal device at the SCADA host site. Communications equipment is transparent
and does not interact with specific remotes; all data is coupled to the host on a single data
line (such a network is commonly used with synchronous radio modems and asynchronous
radio modems).
Figure 2.2 – Point to Multipoint Network
1.1.11 Report by Exception
In a true Report by Exception configuration, the remotes send data to the master only when
an event or exception has occurred in the remote. However, most Report by Exception
systems have a master/remote polling component. The master polls the remotes once every
hour or half-hour to ensure there is still a valid communication path. In a Report by
Exception configuration, there will not be a master controlling RF traffic and RF collisions will
often occur. The Viper has several collision avoidance features to help minimize collisions.
The Viper is a “polite radio”. The Viper will check the RF traffic on the receive channel
before transmitting. If there is no RF traffic present (no carrier present) it will transmit. If
there is RF traffic (carrier present) the Viper will buffer the data. The Viper will transmit the
buffered data when there is no RF traffic present (no carrier present).
EXTENDING THE COVERAGE AREA WITH A RELAY POINT
The Viper has a Relay Point feature that allows a unit to relay data from one RF coverage
area to another RF coverage area. When units are spread over two or more coverage areas,
the user must identify the devices forming the backbone between coverage areas so any
unit can talk to any other regardless of their locations. There can be multiple Relay Points
in the system extending the coverage over several hops.