Cal Amp VIPER SC+ 100 series User Manual - page 193
Viper SC+™ IP Router for Licensed Spectrum PN 001-5008-000 Rev. C
| Page 183
AVOID UNNECESSARY TRAFFIC OVER THE AIR NETWORK
Monitoring Remote PLCs with Monitoring Application Tools
Continuously monitoring remote PLCs for monitoring purposes only via the on-air network adds additional traffic. This
should be avoided unless required. Some of these software tools are made to run on local networks (high bandwidth)
rather than over the air. If used set their timeouts as described above in Messaging with TCP and TCP Connection
Some applications when closed still leave their TCP communication layer running. So even if the main display is closed
background monitoring still occurs. If this is suspected, use Wireshark to capture whether communication persists, or
turn the monitoring PC off temporally to view any impact this has.
Remote Alive Check
Sometimes the PLC could perform pings in parallel to the communication connection. The ping result may be used to
determine the presence of the remote or the master. This should be disabled where possible. If required, change ping
interval to every 5 or 10 minutes. Check with PLC manufacturer for advice.
MESSAGING WITH TCP – OPENING AND CLOSING TCP CONNECTION FOR E ACH POLL
Opening and closing TCP connection for each poll is not recommended. Opening and closing a TCP connection requires
2-3 times more in and out messages then messages for a simple poll. This increases the on-air traffic and adds extra
delays for the polling.
It is best to open all the TCP connections at the beginning when starting the poll and closing the TCP connection when
poll is stopped. Unsolicited messages done at non-regular intervals and more than 4 minutes apart for the same
remote should open and close the TCP connection for each message group.
Having a polling interval of more than 4 minutes for the same remote PLC or having a mixed (more than 4 minutes and
less than 4 minutes) interval for unsolicited messages, the TCP connection should be opened and closed each time.
With the Viper in proxy mode, after 5 to 10 minutes of inactivity, the Viper will remove the internal proxy context and
resume the connection without proxy. Therefore the benefit for proxy is lost.
When opening and closing is required than the additional traffic load for TCP open and closing needs to be planned into
the system design.
SAFE LADDER LOGIC – SUGGESTION
When a PLC remotely controls important operations of another PLC, it would be good to have ladder logic protection in
case of communication failure with remote.
For example, one PLC is at the pump station and the other is at the tank station. To avoid tank overflow in case of
communication loss, it could be possible to design the logic for the pump PLC to detect that if no data message were
received for over 10 minutes from the tank PLC, to turn its pumps off if they were running.
For example the remote PLC inactivity timeout could trigger this or some other method of detection.