PLC Address Changes After Restart? Check How the Address Is Assigned
An address visible today may have been leased rather than stored permanently. If a PLC or adapter moves after restart, inspect its address-assignment mode and server records. Do not assume that entering an address once disabled dynamic assignment.
Addressing behavior is device-specific; coordinate changes with the network owner.
What is happening?
Static configuration, DHCP reservations, ordinary leases, and BOOTP involve different ownership and persistence rules. Device behavior also varies. A reservation may depend on a client identifier rather than the label an engineer expects.
Check these five things
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Read the current assignment mode from supported device configuration.
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Record device identity, current address, mask, and gateway.
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Compare DHCP or BOOTP records with the intended network plan.
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Check replacement-device identity and reservation ownership with the network administrator.
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Validate the approved configuration after a controlled restart.
Worked example
A replacement adapter receives a different lease because its identity differs from the original reservation. Manually changing the HMI connection to that temporary address appears to fix communication but leaves the next lease event unresolved.
Quick diagnostic reference
| Method | Persistence question |
|---|---|
| Static | Stored in device configuration? |
| Reservation | Correct client identity? |
| Lease | What happens at renewal? |
The mistake to avoid
Assigning a static address inside an unmanaged dynamic pool can create conflicts. Address changes require coordinated ownership, not simply choosing an unused-looking number.
How to verify the fix
Confirm address persistence, server records, and all dependent client connections after restart and the applicable renewal test. Update the network drawing and replacement procedure.
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
Cisco: Managing DHCP leases. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.