Fix HMI bezel keypad input failure: check the mounting gap

What each symptom tells you

Work down this list in order; each line sends the fix somewhere different.

• The displayed preset follows an online edit. Connect the PLC and the panel, change the timer preset at V1300 in the programming software, and watch the numeric entry object. A value that tracks the edit tells you the read path, the comms setup and the data point are all good.
• F1 opens entry mode. Select F1 and watch the object switch into data-entry mode. The selection path works, so the fault sits downstream of it.
• The bezel keys change nothing on screen or in the PLC. The keypad input path is the fault. Check the mounting gap before writing a keypad routine in ladder logic or touching an editor setting.
• The keys change the digits but the PLC value never moves. The entry is arriving and the accept or write path is at fault; start from why an HMI reads the PLC but cannot write.
• Nothing works, readback included. That is a power or comms fault, not a keypad fault. Restore the connection first, then come back to the keys.

Check the two paths that still work

Two quick tests tell you whether reseating is the right fix.

1. Repeat the readback. Change the preset at V1300 online and watch the displayed value follow it. If it follows, leave every comms setting alone.
2. Repeat the selection. Select F1 and watch the object enter data-entry mode. If it does not, correct the function-key assignment on the object first; the setup is covered in non-touch HMI numeric entry with function keys.
3. Test the number keys while the object is in entry mode. Outside entry mode there is no value on screen for a numeric key to change, so a test done too early reads as a dead keypad when the keys are fine.
4. Leave the editor settings for last. The editor carries a toggle for sending the current bezel key status, and the project in question ran editor version 2.20.0.0 with that toggle on screen. The keys in the working installation came back from reseating alone, so correct the fit first and open the project settings only if a seated pair of bodies still drops keys.

Seat the display into the bezel

The hardware manual has you cut one opening in the enclosure, pass the assembled display and keypad bezel through it, and secure the assembly with mounting clips. The gap between the two bodies is the weak point in the keypad path, so the fix is mechanical.

1. Put the machine in a safe state and isolate the panel under your lockout procedure before opening the enclosure.
2. Measure the visible gap between the display body and the bezel body at the clips, with feeler gauges if you have them.
3. Loosen the mounting clips evenly and press the display toward the bezel until the gap reads about 1/32 in (0.8 mm). Work around the clips so the parts pull in square, and stop if the parts bind, the case flexes or a clip is damaged. No torque value applies to this joint; the measured gap is the check.
4. Re-tighten the clips, close the panel, restore power and clear the lockout.
5. Retest in order: F1 opens the entry, one numeric key changes a digit, and an accepted entry updates the preset in the PLC software.

Example: reseating brings back a dead 20-key keypad

A 3-inch panel-mount HMI sits behind a 20-key keypad bezel. A numeric entry object edits a timer preset stored at V1300, and F1 starts the entry. The symptoms: the displayed preset follows online edits, F1 opens entry mode, and all 20 keys do nothing.

A feeler gauge at the top clips reads about 2 mm of gap between the two bodies. The target from the working installation is 1/32 in, and the conversion is 1/32 in × 25.4 mm/in = 0.79 mm, about 0.8 mm. The measured gap is roughly two and a half times that, which is why the assembly looks mounted while the keys stay dead.

After isolation, the clips come off evenly, the display seats against the bezel, and the gap now reads about 0.8 mm. With power back, F1 opens the entry, the 5 key puts a 5 in the field, and CLR, 120, Enter leaves 120 in the entry and in V1300 in the PLC. The repair took one lockout and a few clips, and the operators got a one-line briefing on the CLR step the same shift.

Train the clear-before-enter step

This entry style keeps the old digits until someone clears them. The sequence that works: with the object in entry mode, press CLR to clear the displayed value, key in the new preset, then press Enter to accept. Operators who typed a new number and pressed Enter without clearing saw the old preset stay in place, which looks exactly like a keypad fault on a busy shift, so the step belongs on the training card next to the reseating fix.

Later editor releases add a clear-or-not-clear selection in the panel manager, which removes the extra step on updated software; the editor in this project predated that choice. What happens to an accepted entry is covered in moving an HMI numeric entry into PLC memory on Enter.

The decision

Recommendation: when a panel-mount HMI reads its PLC value and the function key opens entry mode but the bezel keys do nothing, isolate the panel and reseat the display until only about 1/32 in (0.8 mm) of gap remains between the two bodies, then retest F1 and one numeric key before changing any ladder logic or editor setting. Add the clear-before-enter step to operator training once the keys respond, and check the editor's release notes for the clear-or-not-clear option before making that step permanent procedure.

If the keys change the entry but the preset in the PLC never moves, the fit was fine and the fault sits in the accept or write path; the symptom list above points to the right article. And when the cell needs the running preset visible from across the floor, that is a separate screen doing a separate job, covered in displaying a PLC counter value where operators can see it and in our shop-floor screens.

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