Use one object with two tag slots. The push button tag writes the Control-Bit the PLC clears on detection; the indicator light tag reads the Stage-Bit that confirms the action. One object on the screen, no stacked layers to keep aligned, and the face reports the machine's answer.
One object can do both jobs. Most panel editors ship a push button with a second tag slot for its own face: the press writes one PLC address, and the appearance reads another. Bind the press to the Control-Bit that asks the PLC to act, and the face to the Stage-Bit that confirms the action happened. The press asks, the bit answers, and you get one object with no invisible buttons and no stacked layers to keep aligned. This page walks the binding, works a three-choice selector down to its object count and label size, and gives the tests that separate a write fault from a read fault. For the simpler job of a plain button driving one output, start with making an HMI button turn on a PLC output through a command bit.
What each bit carries
The two bits carry different news, and the screen should keep them apart.
The Control-Bit is the ask. The press sets it, and the PLC clears it as soon as the logic detects the request. The Stage-Bit is the answer. The PLC sets it when the action has occurred and clears it when the stage ends.
Point the face at the command bit and it lights the instant someone presses, while the machine is still traveling toward the stage. The operator sees a confirmation that was only a request. Web runtimes draw the same loop as pending writes held until the controller confirms them: the request stays visibly unfinished until the real state arrives. Your panel draws it with two tag slots.
| Tag | Who writes it | What it tells the operator |
|---|---|---|
| Control-Bit | The press sets it; the PLC clears it on detection | The ask |
| Stage-Bit | The PLC sets and clears it with the stage | The answer |
What you need
- The screen open in your panel editor, with download rights to the panel.
- The combined object. Editors name it differently; look for an indicator button, or a push button with an indicator tag. It carries two tag slots, commonly labeled a "push button" tag and an "indicator light" tag.
- The two PLC addresses, one for the ask and one for the answer.
- An online view of both bits in the PLC programming software, for the tests below.
Bind one object to both tags
- Add the combined object to the screen and place it where the operator stands. Sizing numbers come with the example below.
- Open its properties and point the push button tag at the Control-Bit address.
- Set the press action to set the bit, in the momentary form, so the PLC owns the bit once it has cleared the request.
- Point the indicator light tag at the Stage-Bit address. This slot only reads.
- Define the two faces, the color or text for 0 and for 1. The two-state pattern is the one covered for showing two text states on an HMI from one PLC bit.
- Download the screen to the panel and run both tests in the next section.
Recommendation: Use one combined object per command, push button tag on the Control-Bit and indicator light tag on the Stage-Bit, whenever the PLC clears the request on detection and reports the result on a separate bit. Keep a stacked button-under-a-light pair only as a patch until the next screen edit.
Example: a three-choice selector read from 4 ft
Inputs. A mode selector with 3 choices, each needing one command press and one confirmation light. The operator stands at the panel, 4 ft from the glass, a typical read distance for a machine-mounted screen. Legibility planning uses 30 ft of viewing distance per inch of capital height (FHWA); 20 ft per inch is the figure for fast reading, so the arithmetic uses 20.
Rule. Divide the viewing distance by 20 ft per inch to get the minimum capital height. Count one object per choice, plus one label for the group.
Arithmetic. Label height: 4 ft ÷ 20 ft per inch = 0.2 in of capitals, rounded up to 0.25 in. Built from separate buttons and lights: 3 choices × 2 objects = 6, plus 1 group label = 7 objects, carrying 3 layered pairs. Built from combined objects: 3 × 1 = 3, plus 1 label = 4 objects, carrying 0 layered pairs.
Result. Four objects, none overlapping, labels at 0.25 in capitals. A floor-level board read from farther away scales by the same rule: at 20 ft the capitals need 20 ft ÷ 20 ft per inch = 1 in. If the whole line needs to see the active stage, hang a larger board beside the machine; our shop-floor screens run offline playback or your own Android app.
Decision. Any choice that needs both a press and a confirmation gets one combined object. Count the layered pairs on a screen: every pair above zero is rework waiting for an edit window. The one-of-three rule lives in the PLC logic, and if two stages can run together in that logic, two lights will show together. Fix the interlock in the controller and let the screen report the bits as they are.
Check the ask and the answer separately
Put the machine in a state where an unexpected stage change harms nothing, then test one path at a time.
The write path. Press the object and watch the Control-Bit in the online view. It should jump to 1 and drop to 0 as the logic detects it. A bit that never moves points at the push button tag address first, then at a password on the PLC that blocks writes while reads still work, a case explained in why a PLC password can block HMI writes but allow reads.
The read path. Run or force the machine into the stage and watch the Stage-Bit. The face should change only when the bit changes. A flat face over a moving bit points at the indicator tag, and the way through it is the value trace described for HMI indicator objects that ignore tag state.
The cross-check. Press and hold with the machine unable to complete. The face should hold the unconfirmed state until the Stage-Bit moves. A face that lights on the press alone has its indicator slot on the Control-Bit; swap the address.
When the editor gives you no combined object
Some packages lack the two-slot object or bury it. The workable patch is stacking in the right order: place a visible push button first, then drop the indicator light on top of it. The light covers the face and the press still lands on the button underneath. Check that with the write-path test: press where the light sits and watch the Control-Bit. Laying out the pair as separate objects in the first place is covered in wiring an HMI button and indicator to PLC bits.
Treat the stack as a patch. The two objects can drift apart in later edits, and the touch test has to be repeated after every edit. Keep the command tag and the status tag on their own objects even in the patch, so the later swap to a combined object is two property edits. Retest both paths after the swap.
Questions
Should the command side be a momentary or a maintained button?
Use the momentary form when the PLC clears the command bit on detection, as it does here: the press writes, the release stops writing, and the PLC owns the bit from there. A maintained button holds its state and keeps driving its tag, which suits bits the logic expects an operator to set and leave set.
My editor has no object with two tag slots. Is the stacked patch acceptable?
For a while, yes. Place a visible push button first, drop the indicator light on top, and check that the press still moves the command bit. Select both objects and group them if your editor supports grouping, so a later edit moves them as one. Replace the pair with a combined object at the next controlled screen update.
Two of my three selector lights show together. Is that a screen fault?
Look in the PLC first. The screen mirrors the Stage-Bits as the logic sets them, so two lights on tells you two bits are high, or one indicator slot points at the wrong address. Check the tag addresses on the objects, then ask whether the logic that sets each Stage-Bit can be true at the same time. Fix the interlock in the controller.
Does the lit face tell me the machine reached the stage?
It tells you the Stage-Bit is 1. The bit is as truthful as the logic that sets it. Open that logic and see what drives the bit: a field signal such as a limit input makes the light a real confirmation, while a coil set from the command side only redraws the ask on the face. Run the check with the machine held in a safe state.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.