CommonNetworkIssues
Q: My LAN is slow and PLCs can’t seem to connect. How can I quickly
isolate the problem and get to root cause?
A: The inability to connect tends to be a point-source problem. Keep
in mind that the PLC must have connected properly at least once
before or the problem is approached as a new installation instead of a
troubleshooting event. The best new install approach is to certify the
infrastructure. The best troubleshooting approach is the “divide and
conquer” strategy. Assuming that this is an IP-based installation, there
are a minimum number of requirements for normal PLC communications.
If Ethernet Link state is not established, check for:•
Bad cables -
Ethernet errors on the local link -
Failed, marginal, blocked, or misconfigured switch ports -
(including accidentally VLAN tagged ports)
Failed, marginal, or miscongured PLC Ethernet interface -
Most problems of this nature are identified quickly by connecting
EtherScope Network Assistant in place of the PLC using the PLC’s
Ethernet cable. The cable may also be reversed so that EtherScope tests
the PLC Ethernet interface and attempts to establish link.
If a valid IP address for the local subnet is not present, check •
the following:
Ensure that any statically assigned IP address on the PLC is -
correct for the local subnet, including subnet mask and default
router.
Verify that a DHCP (or BOOTP) address may be obtained from -
the PLC’s Ethernet connection using either the PLC or
EtherScope. If the local switch port has been accidentally
congured as a VLAN tagged port then this will fail.
EtherScope will report the presence of tagged trafc.
Ping each server which the PLC communicates to over the PLC’s -
Ethernet connection using either the PLC or EtherScope.
If these tests all pass, then reconnect the PLC and see if it is now work-
ing. Sometimes a cable is not fully inserted or the switch port is reset
by dropping link state. If it begins working, then double-check the
local cable connections carefully. It may be a poorly crimped RJ45, or
the RJ45 connector is designed for solid conductors instead of stranded
(or the reverse) and has become intermittent. Check also for water or
other contamination of the Ethernet port or water ingress in the link.
The most common reasons for slow or poor performance include
overloaded or underpowered servers, unsuitable switch or router
configurations, traffic congestion on a low capacity link, and chronic
frame loss. The EtherScope can help determine which of these reasons
is causing performance issues.
Q: My connection continues to be dropped. What could be
causing this?
A: The reason for dropped connections is a logical or physical
connectivity loss. The EtherScope Network Assistant can help you
determine which of these problems is causing the dropped connection:
Errors on the ports which are used in the path•
Duplex settings•
Spanning tree problems•
Routing problems•
Overloaded WAN link or a marginal interface in the WAN path•
EMI or other electrical of RF interference•
Q: I suspect unauthorized devices have been added to the network.
Is there an easy way to verify this?
A: The EtherScope Network Assistant helps you to proactively baseline
your network to register devices on the network. This allows you to
know which devices are normally there, then when the reactive/tactical
discovery reveals a list of currently attached hosts you would recognize
anything inappropriate.
TroubleshootingCommonIndustrialNetworkIssues
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