PLC Network Drops Randomly? A Duplicate IP Can Look Like Bad Hardware
Intermittent access after adding or replacing a device should put duplicate addressing on the investigation list. Two endpoints claiming the same IP address can make the connection appear to move between devices. Confirm identity as well as reachability before replacing a switch or controller.
General Ethernet diagnostic reasoning; investigation requires authorized network access.
What is happening?
On an Ethernet subnet, IP communication relies on resolving an address to a link-layer destination. Conflicting device identities can produce misleading symptoms: a web page changes, an engineering connection drops, or diagnostics identify a different product. These observations are clues, not proof without corroborating network evidence.
Check these five things
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Record when the problem began and which devices were added, restored, or connected for commissioning.
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Compare the approved IP inventory with the addresses actually assigned to the affected equipment and engineering laptops.
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Have an authorized network administrator inspect address-resolution and switch information for the suspected address. Record device identities over time.
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Review DHCP reservations, static assignments, and replacement-device procedures. A restored project can reintroduce an old address.
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Resolve the conflict through the site's change process, then update the asset record and replacement instructions.
Worked example
A technician sometimes reaches the intended drive and sometimes sees a different device's identification page at the same address. That observation warrants an identity check. It does not justify disconnecting random production cables until the page stops changing.
Quick diagnostic reference
| Clue | Evidence to collect |
|---|---|
| Started after replacement | New device configuration |
| Identity changes | MAC and device identification |
| Only laptop connected causes fault | Laptop address assignment |
The mistake to avoid
Ping results alone cannot prove uniqueness. A single successful reply may hide the fact that another device can also claim the address.
How to verify the fix
Confirm stable device identity and normal communication over a representative operating period. Check that the corrected allocation survives the documented restart or replacement process.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Rockwell Automation: EtherNet/IP network troubleshooting. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.