Honeywell UDC UNPLUG Error Message

People will fire up a new UDC 1/4 DIN loop controller, like a UDC2500 or UDC3200, and discover that the lower display periodically flashes an error code: UNPLUG. But what does it mean? Is something wrong?

A search of the pdf version of the manual hints that the Modbus Ethernet communications is involved:

The Ethernet status screen shows the network status of the Ethernet Link. This may be accessed either via Ethernet or via Infrared Communications. For example, if the Ethernet cable is unplugged, then the instrument cannot send up the EUNPLGED diagnostic message via Ethernet.

But the word UNPLUG is nowhere else in the manual.

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