Why does my Honeywell UDC controller flash UNPLUG at me?

Honeywell UDC Controller.jpg

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.

The UNPLUG code is telling you that the controller has an Ethernet card but that the Ethernet port is not connected to an Ethernet device (switch, router or other). Just plugging a cable into the UDC without an active Ethernet port connection on the other end will not make the error disappear; it is looking for an active port connection.

In terms of diagnostics and error messages, this one’s only really critical if you’re using Ethernet communications. It tells you that you have a connection issue, which is usually pretty easy to solve.  Otherwise, it’s just an annoyance that takes us display screen space.

If you’re not going to be using the Ethernet communications built into your controller, you can disable the error code by navigating in setup to the COM group, and at the ComSTA parameter, changing it from ETHR to DIS (disable).   The spelling of the prompts will change slightly on the UDC 3200 and 3500 models.

Lesman Instrument Company is the authorized Honeywell representative in Illinois, Indiana, Wisconsin, Eastern Iowa, and Michigan’s Upper Peninsula. Learn more about these products at Lesman.com or get technical assistance by calling 800-365-8766.


Dan Weise, Lesman Product SpecialistDan Weise, Lesman product specialist, is an instructor for training classes on process instrumentation hardware, software, and technology.

Dan has been involved in all facets of data acquisition and process instrumentation since 1978, from sales and commissioning to service and support. He’s a long-time member of ISA, and has been with Lesman since 1988.

In his words, Dan’s the guy “who reads all the manuals nobody else reads”. In Lesman customers’ words, he’s the trainer to call if you want to “cut to the SO WHAT of instrumentation”, so it’s easy to understand.

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  1. #1 by Paul statler on May 6, 2021 - 11:03 am

    But what does it mean when at flashes “input2 fl” in conjunction with that

    • #2 by danstips on May 6, 2021 - 3:53 pm

      Dan says: The second input is enabled but is not producing a valid reading, probably an open circuit.

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