User's Manual

001-5008-000 rev5_2.doc Page 18
2.3.1 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
2.3.2 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 ViPR has several collision avoidance features to help minimize collisions.
The ViPR is a “polite radio”. The ViPR 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 ViPR will buffer the data. The ViPR will transmit the buffered
data when there is no RF traffic present (no carrier present).