5. Hardware swap detected on login

Symptom

Login succeeds, but the POISerialNumber in the LoginResponse differs from the value your POS stored.

What it means

The physical terminal at this lane changed — replacement after failure, units rotated between lanes, or maintenance swap. The protocol works fine, which is exactly why your POS must notice: nothing else will.

Correct handling

1. Do not silently overwrite the stored serial. That defeats the check's purpose.
2. Surface the change to the merchant/supervisor: "terminal replaced — confirm". A deliberate swap gets confirmed in seconds.
3. On confirmation, persist the new serial and resume normal operation.
4. If the swap is not recognised by staff, stop transacting on that lane and raise it — an unexplained device change on a payment lane is a security event.

Operational notes

• Terminal replacements often arrive with different software versions; after confirming the swap, run Diagnosis and check POISystemData before the first sale.
• Multi-lane estates: make sure the stored serial is keyed per lane (per POIID), or lane rotations will trigger false alarms.