HMI numeric display shows nothing: check the tag address first

What a blank numeric object means

The display path is a chain. The PLC program writes a value into a data word. The HMI tag reads that word with a data type that matches how the value was stored. The numeric object shows the tag's value. A fault at any link gives an empty, stale or nonsense number, and the links fail for different reasons: the link between panel and PLC, the mapping inside the tag database, and the object's own limits and assignment.

Communication status covers only the first link. A panel can exchange data with the PLC without a single error while a tag points 8 words away from the temperature, because the tag is still reading a real word. When the PLC side needs checking too, the why an HMI shows nothing while the PLC still runs page lists the panel-level causes.

Symptoms and what each one points to

Match the symptom before you open any configuration:

• The whole panel is dark and every object is gone. Power or panel hardware. Work through HMI screen goes blank and freezes: check power first before touching tags.
• One object is blank and the objects around it update. The mapping fault this article works through: the tag's device, address or data type.
• One object is blank and the panel's diagnostic list shows communication errors. Clear the link errors first. The blank object follows the link until the errors are gone.
• The object shows a number that never moves. The tag reads a word the program never writes, often an address one word off or copied from an old project.
• The object shows a nonsense number. A representation mismatch: BCD read as binary, signed read as unsigned, or half of a two-word float.
• The object blanks at some process values and reappears at others. The value is crossing the object's minimum or maximum limit.

Watch the live PLC word first

Go online with the PLC programming software while the process runs. Find the rung that scales the analog input and note the destination word of its scale or move instruction. Open that word in the data monitor; the data-word area is called V-memory on some PLC families, data registers or holding registers on others.

Watch the word while the temperature changes. A 12-bit analog input stores raw counts from 0 to 4095, and the scaling instruction turns counts into engineering units, commonly tenths of a degree, so 68.3 °F sits in the word as 683.

Two outcomes:

• The word moves with the temperature. The PLC side checks out and the fault sits in the tag or the object. Go to the tag comparison.
• The word holds still through the change. Troubleshoot the input card and the scaling rung first, because no tag change can display a value the PLC never writes.

Compare the tag with the word you watched

Open the tag in the tag database and compare three entries against the word you watched:

1. Device. The tag must point at the PLC your program runs in. A device entry left over from a copied project sends the tag to a controller that is not running your process, while the link to the real PLC stays clean.
2. Address, in the number base the field expects. Address entry fields take different number bases between PLC families and software. A value typed in the wrong base selects a different word: 40 typed in octal selects word 32 decimal, because 4 x 8 + 0 = 32. Check the base before trusting an address copied from another project.
3. Data type, matched to the stored representation. A 16-bit signed integer holds -32768 to +32767 and an unsigned one holds 0 to 65535. BCD packing and 32-bit floats spread over two consecutive words are also common. Pick the type from how the scaling instruction stores the value.

The representation mismatches each look different on screen. A BCD-coded word shows a nonsense number when the tag reads it as plain binary; the BCD analog register fix walks that case. A temperature that dips below zero shows a huge positive number when a signed value is read as unsigned; the signed decimal data format fix covers that setting. A float read from only one of its two words gives garbage; the two-word float display setup shows the word arrangement. If the panel raises invalid address errors while you tag PLC memory, the invalid address fix covers that message.

Scaling is decided once, in the PLC. If the program stores tenths of a degree, the object needs one decimal place and no scaling of its own; a value that lands ten times too large after a scaling change is the separate fault covered in the ten-times-too-large trend fix.

Worked example: a 68.3 °F tank tag landing 8 words low

Inputs, with where each comes from: the program stores tenths of °F in one 16-bit signed word and the tank holds 68.3 °F, so the true word value is 683 (tenths storage is the common scaled-integer practice; the process value is illustrative). The intended address is word 40, the destination of the scaling instruction (illustrative address). The address was typed as 40 into a field that expects octal (the number-base mismatch, the documented failure mode). Neighbor word 32 holds raw input counts, currently 2730 of the 0 to 4095 span of a 12-bit input (illustrative count inside the real span). The object was drawn with minimum 32 and maximum 212 for the °F display range (illustrative limits; the out-of-range blanking is the documented behavior).

The rule in words: an address typed in the wrong base selects the word those digits mean in that base, and a numeric object hides any tag value outside its minimum and maximum limits.

The arithmetic: octal 40 = 4 x 8 + 0 = 32 decimal, so the tag lands 8 words below the true word and reads 2730 counts. 2730 is above the 212 maximum, so the object displays nothing. The link stays clean because the panel is reading a real word, only the wrong one.

The test: remove the minimum and maximum limits on the object and apply. The object now shows 2730, a plausible-looking count with no temperature meaning. That result puts the fault in the address, because a correct tag would show 683.

The fix and its result: retype the address as 40 in the base the entry field expects, restore the 32 to 212 limits, and set one decimal place for the tenths scaling. The object shows 68.3 °F and tracks the tank.

The decision rule: if removing the limits reveals a number of the wrong magnitude, counts near the 0 to 4095 span instead of a process temperature, fix the address; if removing them reveals the correct temperature, the limits themselves were the fault and the tag needs nothing.

Correct one thing, then test three points

Make the change the comparison identified, one entry at a time: the address, or the data type. Two changes together hide which one fixed the display. Apply the change through the project's normal download and note what you changed, so the project can go back if a test makes things worse. Leave the PLC program untouched during this; a scaling rung and a tag edited together create two new values to explain.

Then test three points while the process drifts a few degrees:

1. The live word in the PLC data monitor.
2. The tag's live value in the HMI diagnostics, where the software shows one.
3. The number on the object.

All three must move together. A live word with a dead tag value points back at the tag entry; a live tag with a blank object points at the object itself.

When the tag is right and the object stays blank

Tag assignment. Open the object and confirm it is bound to the tag you just tested. A similarly named tag, or a deleted tag left behind after an edit, binds the object to a value nobody updates.

Minimum and maximum limits. Remove the limits temporarily and apply. Some HMI packages hide a tag value outside these limits, and removing them temporarily is the usual check for that behavior. If the value reappears with the limits gone, widen the limits to cover the real process span and put them back.

Visibility conditions. An object can be correct and hidden: behind a shape drawn later on the same screen, or behind a visibility condition tied to another tag. Toggle the condition or the layer order and look again.

The order to work in

Recommendation: before touching any display setting, go online in the PLC software and watch the destination word of the scaling instruction while the temperature changes. If that word moves, correct the tag's device, address and data type against it, one change at a time, then test at three process values before cleaning anything else up. If the word holds still, the fault is upstream in the input or the scaling rung and the panel needs nothing.

More checks for shop-floor screens, from choosing and mounting a panel to ghost touch at high temperature, are collected in the shop-floor screens and andon boards category.

Questions