Viper SC+™ IP Router for Licensed Spectrum PN 001-5008-000 Rev. C
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Figure 97 – Typical PLC Setup Channel Configuration Parameters
When the polling interval is longer, CIP Forward Open and CIP Forward Close messages add 4 extra messages on-air
between each unit polled. In each poll is 2 messages (message and reply), therefore the 4 extra messages increase the
on-air message load by 200 %.
For example, a system is set up for the PLC to poll the remote PLCs every 120 seconds and is set to wait for the next
poll loop if polling is not completed after the 120 seconds, therefore the loop becomes 240 seconds.
Take the 240 seconds and divide it by 8, which gives 30 seconds. Set the Message Reply timeout on Channel 1 to 30000
milliseconds — or 32000 ms is some margin is required.
More about Message Reply Timeout
The Message Reply timeout is also used for retransmission of messages in case there is no reply. Since TCP connection
is used, the retransmissions are normally not required. Therefore with TCP longer timeouts within reason are okay.
The Message Reply timeout on Channel 1 settings of the PLC should be set to the value determined by the previous
example and since traffic is on-air and is retried, and the TCP driver performs its own retries, then:
Msg Reply Timeout minimum = 10000 msec.
Msg Reply Timeout maximum = based on value determined by the above example.
With future releases of PLC software and firmware, the described operation could change. It is always recommended to
be informed on PLC release changes from your PLC provider or manufacturer or Allen-Bradley or Rockwell Automation
support.
PLC Sends Many TCP/IP Keep-Alive Messages
This has been seen on the MicroLogix 1100 and 1400 and on the SLC 5.