Fix an HMI numeric keypad that is too small on a high-res panel

Why the keys stay small

Two causes produce a small keypad, and they need different responses. The first is scaling. A keypad sized in pixels keeps its pixel count, and a pixel gets physically smaller as the panel packs more of them per millimeter, so the same keypad setting covers less glass on a high-resolution panel of the same diagonal. One HMI vendor's published documentation states it plainly: the target device scales the Large keypad and keyboard to fit the screen. The option can be honored and the keys can still land under a fingertip.

The second cause is a project that never reached the panel in full. The editor preview renders at the editor's own scale, so the glass on the panel is the only reference that counts. The checks below separate the two causes.

Symptoms and what each one points to

• Keys small in the editor and on the panel after a full download. The option is honored and the scaled size sits below your target. Measure, then build the pop-up keypad.
• Keys measure the same with the option off and on. The option has no effect on this panel and software pairing, so record the versions, check the release notes, and run the pop-up keypad meanwhile.
• Keys look small only in the preview. The preview renders at its own scale. Judge the panel glass.
• One field draws a small keypad, the rest draw a big one. Some packages set keypad style per entry field. Open that field's own properties before blaming the project option.
• A key never registers at all. That is a touch fault, not a sizing problem; see why a keypad can stop responding to keys.

Checks before you change anything

1. Write down the programming software version from its about dialog and the panel firmware version from the panel's system information screen.
2. Open the project attributes, General screen, and check that Large numeric entry is ticked. That is the option that sizes the built-in numeric keypad.
3. Save and download the full project. A partial or cached transfer can leave the old keypad in place.
4. Power-cycle the panel, open a numeric entry field, and let the keypad pop up on the glass.
5. If it still looks small, pick up a ruler. Ticking and re-saving the same box a second time changes nothing else.

Measure a key on the glass

Published touch research puts the minimum button for a fingertip at 10 mm, and measured work with older users lands at 11.43 mm. For operators with reduced dexterity, the published recommendation climbs to 19.05 mm buttons with 6.35 mm to 12.7 mm of spacing between them. Floor practice on many sites runs down to about 8 mm square; treat the 10 mm figure as the target and 8 mm as the absolute floor for a bare finger. Test the finished pad with the thickest gloves on your floor, because reduced dexterity is exactly the case the larger figures were measured for.

Read the measurement like this:

• At or above the minimum with the option on: keep the built-in keypad.
• Below the minimum, and the size changed between the two downloads: the option works but lands too small. Build the pop-up keypad.
• Below the minimum, and the size never moved: check the release notes for a scaling fix, send the vendor the versions, and run the pop-up keypad meanwhile.

Example: sizing a 4 by 4 keypad on an 8-inch panel

Inputs, and where each comes from: an 8-inch high-resolution panel, the panel class where this problem bites; a 10 mm minimum key from published touch research; 6.35 mm between keys, the tight end of the 6.35 mm to 12.7 mm published spacing range; and a 4 by 4 grid holding digits 0 to 9, Clear, Backspace, Enter, a decimal point, a minus sign, and one cell to spare.

Rule in words: a grid's width is the number of columns times the key size, plus one gap between each pair of columns. Height works the same way by rows.

Arithmetic: width = height = 4 × 10 mm + 3 × 6.35 mm = 40 mm + 19.05 mm = 59.05 mm.

Result: the key grid needs about 60 × 60 mm of free glass.

Decision: lay the pad out at minimum-size keys and 6.35 mm gaps whenever the free area gives you about 60 × 60 mm. When it does not, drop to a 3 by 4 grid, digits 0 to 9 plus Clear and Enter, with Clear doing the delete work: width = 3 × 10 mm + 2 × 6.35 mm = 30 mm + 12.7 mm = 42.7 mm, height 59.05 mm. Shrink the grid, never the button; every key stays at or above the minimum.

Build the pop-up keypad

Draw the pad on its own screen and open it over the process screen; the mechanics are in build an HMI pop-up screen with a screen window. Give digit keys faces at the minimum size, and give Clear, Backspace, and Enter wider faces and wider gaps, because mis-hits concentrate on those three.

Route the digits through a scratch register. Declare integers entry = 0, digit_count = 0, and max_digits = 3, range limits lo = 0 and hi = 250 for this example setpoint, a boolean range_error = 0, and the destination tag dest as the only write target:

• Digit n pressed, while digit_count < max_digits: entry := entry * 10 + n, then digit_count := digit_count + 1. Ignore the press once digit_count reaches max_digits.
• Clear pressed: entry := 0, digit_count := 0.
• Backspace pressed: entry := entry / 10 with integer division, and digit_count := digit_count - 1, floored at 0.
• Enter pressed: when lo <= entry and entry <= hi, write dest := entry; otherwise set range_error and leave dest alone.

Enter is the only write to dest, and the range check is the part that keeps a mistyped 999 off the machine. That commit-on-Enter discipline is the same one behind a delayed write commit timer for HMI switch presses, and the limit comparison itself is laid out in clamping HMI numeric entry limits in PLC ladder logic.

Test it on the panel: enter 0, then 250, and confirm dest takes each value after Enter. Enter 251 and 999; both must leave dest unchanged and set range_error. Cancel a half-typed entry and cycle power mid-entry; in both cases dest must still hold the last committed value.

Recommendation: keep the built-in keypad only when a key measures 10 mm or more on the glass after a full download; below that, build a pop-up pad with minimum-size keys and 6.35 mm gaps, about 60 × 60 mm for 4 by 4 or 42.7 × 59.05 mm for 3 by 4, feeding a scratch register and committing to dest on Enter only after the range check passes.

One duty note for the hardware: manufacturers commonly rate industrial touch panels around 1 million touches at normal finger pressure. A setpoint pad pressed a few dozen times per shift sits far below that. A pad that serves as the main keypad for every operator, every shift, is the case where machine design guidance, including NFPA 79, points you at physical pushbuttons.

When to stop troubleshooting

Stop when a full download, a power cycle, and current matched software and firmware still leave the built-in keypad unchanged with the option on. Send the vendor your recorded versions, the panel resolution, and both ruler measurements, and keep the pop-up pad in service while you wait.

The permanent repair is one of two things. A release in which the large-entry option scales correctly on your panel: repeat the ruler measurement to confirm it. Or a documented confirmation that the option is ignored at your panel resolution: then the pop-up pad becomes the standard method, and the project notes record the versions and measurements behind the decision.

One shortcut to refuse: lowering the project resolution to inflate the keypad. Unless the panel's documentation supports that mode, the result is a blurred or mis-scaled image, so check how the project fits the panel before you change resolution in any direction.

Questions