How PLC-driven HMI graphics represent equipment states

What changes the picture

A state-selected graphic has three parts: a source value, a mapping, and an image object. The PLC supplies the value. The HMI configuration maps that value to an image. The screen object draws the selected image.

For a two-state indication, a Boolean tag can select between two pictures. For several states, a numeric tag can select from a larger image set if the HMI supports that function. Depending on the terminal, this may be called an indexed image, a state graphic, or indirect image selection. Use the terminal documentation for the supported object and addressing method.

A fixed symbol has no changing selection source. Drawing a fan in a symbol editor does not make its blades rotate. You must configure either state selection or a supported animation property.

Keep these two purposes separate:

• State indication: The picture changes because the reported equipment condition changes.
• Motion effect: The picture cycles through frames to suggest movement while a particular condition remains active.

A valve changing from a closed symbol to an open symbol is state indication. A fan cycling through blade positions while running is a motion effect. Neither proves physical movement unless its enabling signal represents suitable feedback.

Decide whether the graphic shows a command or feedback

Choose the meaning before choosing the artwork. A PLC command, an electrical output, and physical equipment feedback answer different questions.

A start command means the control logic has requested operation. An output state means the logic is commanding an output channel. Neither establishes that power reached the load, that the mechanism moved, or that the process responded.

Feedback also needs a precise label. A drive-ready signal is not the same as a speed indication. A valve coil command is not the same as an end-position switch. Pressure downstream of a valve may indicate a process condition without directly confirming valve position.

Use labels that match the available evidence:

• “Start requested” for a start request.
• “Run command active” for an active run command.
• “Rotation detected” for suitable rotation feedback.
• “Open position confirmed” for feedback intended to confirm that position.

If both command and feedback are available, show them separately or use clearly defined intermediate states. For example, show “Open requested” while waiting for open-position feedback. Do not immediately draw the valve as confirmed open just because the command changed.

The absence of running feedback also does not automatically prove that a fan has stopped. It may mean stopped, below the detection threshold, unavailable feedback, or another condition defined by the control design.

Define the state map before drawing images

Write a small state table before configuring the screen. This gives each image a meaning that can be checked independently of its appearance.

The following valve example uses illustrative values, not a prescribed register layout or terminal image-number range:

The control design must define how these states are produced. Do not infer an end position merely because the opposite position switch is inactive.

Define precedence too. If a discrepancy condition and a command are both active, decide which state takes priority. If both end-position inputs appear active, do not let whichever condition happens to be evaluated last silently determine the picture. Handle combinations according to the documented equipment and sensor behavior.

Give unexpected numeric values an explicit fallback. A blank area or an unrelated image can hide a mapping error. Also define how the HMI indicates unavailable or stale communications data. An out-of-range value and an old but otherwise valid value are different problems.

Keep every image the same canvas size and align the equipment outline consistently. Otherwise, switching states can make the graphic appear to jump even when selection is correct. Pair color with text or a distinct shape so color is not the only cue.

Keep animation timing separate from equipment state

A rotating fan effect normally uses several blade images displayed in sequence. The running indication enables the sequence; an animation index chooses the current frame.

Keep those values separate. The equipment-state tag might remain “Rotation detected” while the frame index changes repeatedly. This prevents a decorative frame number from being mistaken for a new machine state.

Where the HMI supports local animation, it may be possible to generate the frame sequence in the display rather than repeatedly changing a PLC image-selection value. Where indirect selection is required, use the terminal's documented method and a defined sequence of valid image numbers.

Do not advance frames on every PLC scan and assume every frame will appear. PLC execution, communication updates, and screen refresh do not occur at the same rate. The HMI may receive only some intermediate values.

Choose an animation pace for recognition, not as an implied measurement. A blade animation should not suggest measured shaft speed unless that relationship has been deliberately implemented and validated. A steady symbol with a clear running label can communicate the condition better than a fast sequence.

Trace the signal when the picture is wrong

Check the indication in the same order that information reaches the screen:

• The source state is wrong: Review the command or feedback conditions used to derive it. Changing the artwork cannot correct an incorrect source meaning.
• The source changes but the selected value does not: Inspect the logic that creates the display-state value, then check its address and data type in the HMI.
• The selected value changes but the image stays fixed: Check whether the object supports state selection and whether its selection property is connected to the intended tag.
• The image changes to the wrong state: Compare each value with its assigned image. Check for reversed Boolean meanings or incorrect numeric assignments.
• The graphic jumps or flickers: Check state precedence, rapidly changing inputs, and image alignment before adjusting animation timing.
• The picture remains plausible after communication is lost: Review data-quality handling and the stale-data indication. A retained picture is not fresh confirmation.

Observe the source tag, derived display value, and selected image together. This separates a source-logic problem from an address problem or an image-mapping problem.

Avoid adding delays simply to make a confusing picture look stable. First establish whether the underlying signal should change rapidly and whether its handling belongs in the control logic or the presentation layer.

Check meaning as well as image selection

Validate the mapping in an isolated simulation or through the site's approved safe commissioning procedure. Do not force physical inputs, bypass interlocks, or energize equipment merely to demonstrate an animation.

Check every intended state, the return transitions, unexpected values, startup before a valid update, and communication loss. Confirm that unavailable information looks different from a confirmed stopped or closed condition.

A useful acceptance record contains the source condition, the expected display value, the expected image and label, and the observed result. Include any mismatch between a command and its feedback. Seeing every picture once is not enough; each picture must appear for the correct reason.

Finally, keep indication separate from control authority. The distinction between a factory status display and an operator HMI matters when the interface also sends commands. An animated picture is an indication, not a safety function, and it must not replace the machine's required protective measures.

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