User's Manual

001-5008-000 rev5_2.doc Page 17
capture the airwaves to transmit. The RTUs will not respond in the order they were polled
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 ViPR has the ability to use two completely different and separate SCADA polling
protocols, it is important to have interaction between the two protocols. The ViPR 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.
2.3 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 ViPR radio modem.
Serial connections are transparent pass-through connections, allowing the use of legacy
serial devices in the ViPR product environment.
Figure 2.1 – Point-to-Point Network