Viper User
Manual (001-5008-000 Rev6)
Page
17
but will respond when they are ready and have captured the airwaves. The dominant RTU is
created because it happens to reply at just the right time and be in the right order in the
polling sequence.
A common method for a polling master to manage RF traffic is for the HMI/PLC polling
master to poll one remote at a time. The next polling message is not sent until the current
message has been completed (“Done”) or has timed out. This prevents more than one
outstanding polling message. Ladder logic programs typically refer to these parameters as
the message “Done” and “Error” bits. The “Done” and “Error” bits parameter values can be
adjusted for longer timeout values, if required.
Because the Viper has the ability to use two completely different and separate SCADA
polling protocols, it is important to have interaction between the two protocols. The Viper
can send out an Ethernet TCP/IP polling message and also an RS232 polling message, which
may or may not be generated by the same HMI/PLC. CalAmp recommends the user
program the polling sequence in each protocol with logic that interacts with the other’s
protocol “Done” and “Error” bits. The Ethernet polling protocol would not be allowed to
send a message until the current Ethernet message is either “Done” or “Error”
and
the
previous RS232 message are either “Done” or “Error” bits are set. The RS232 polling
protocol would also have a similar logic.
POINT-TO-POINT
A point-to-point network is the most simple of all networks, and may be used for connecting
a pair of PC's, a host computer and a terminal, a SCADA polling master and one remote,
mobile applications (like in-vehicle GPS receivers and base stations) or a wide variety of
other networking applications.
System configurations indicated above allow for either Ethernet or serial interfaces. In
bridge mode, all the network devices are on the same IP subnet. In router mode, the
Ethernet connection on the polling master unit and the remote(s) use different IP subnets.
A hub or switch may be used to allow multiple devices to connect to the Viper radio modem.
Serial connections are transparent pass-through connections, allowing the use of legacy
serial devices in the Viper product environment.
Figure 2.1 – Point-to-Point Network
