The LED on a fixed-function operator panel is driven by the panel's own firmware and lights only while the button is held, so no program can hold it on. Keep the button momentary, keep one retained pause state in the controller, toggle it once per press with edge detection, and show pause with a separate indicator driven from that state bit.
On a fixed-function operator panel, the red LED built into a pushbutton belongs to the panel, not to your program. Panel firmware lights it while the button is held and puts it out the moment the button releases. There is no program interface to that LED, so it can never show a pause that started at another station or outlasted the press. One honest path remains: keep the button momentary, keep one retained pause state in the controller, toggle that state once per press with edge detection, and light a separate indicator from the state bit. This page shows how to tell press feedback apart from state indication, why a held press can flip your state hundreds of times without edge detection, and which checks to run at every pause station before you hand the machine back.
What the LED is connected to
Panels of this type give each fixed pushbutton a toggle or momentary mode, and the mode decides the contact behavior and the LED with it. In momentary mode the firmware turns the LED on for the duration of the press and off on release. That is press feedback. It answers one question, "is someone holding the button right now", because the panel has no input through which the program could write a state to that lamp.
The distinction matters on a machine with several pause buttons. The pause condition lives in the controller. A lamp driven by one button's press can never show it: another station can pause the machine while this button sits released, and the program can resume it with no button pressed at all.
Symptoms and what each one points to
Work down this list from the top; the first match is usually the cause.
- The LED lights only while the button is held, and the machine stays paused after release. The logic already retains the pause state and the panel LED is press feedback. The machine is right, the indication is wrong.
- The LED goes dark and the machine resumes on release. Pause is only commanded while the button is held. The program has no retained state yet, so fix the logic before touching indication.
- The controller input stays true after release. The panel button is set to toggle mode, or the input is mapped to the wrong address. Check the mode setting and the input mapping first.
- One press flips pause and run back and forth, or a held press cycles them. The program toggles on the input level, once per scan. It needs edge detection; the worked example below shows how fast that adds up.
- A new light follows one button but not the others. It is wired to that button's input. Move it to the shared pause state bit.
Check the button mode and the controller input
Confirm the panel button is configured as momentary. Watch the controller input while you press and release: a momentary input follows your finger and drops when you let go. Release the button with the machine paused and confirm the pause condition holds. If it holds, the state is already retained and the mismatch is only the lamp.
Then operate each of the other pause buttons in turn and read the same pause bit after each press. If every press changes one shared bit, a single indicator can serve every station. If the buttons write separate logic, merge them into one retained state first; two pause states on one machine is the deeper fault.
Example: a 2-second hold on a 10 ms scan flips the state 200 times
Inputs: the operator holds the pause button for 2 s, a normal held press; the controller evaluates this rung every 10 ms, typical for a small machine program; the machine starts in run. These two numbers are example inputs, not measurements from your line. Time your own scan and a few real presses before you trust the arithmetic.
Rule in words: a level-triggered toggle changes the pause bit on every scan while the input is true. An edge-triggered one-shot changes it once, on the scan where the input rises from released to pressed.
Arithmetic: 2 s is 2000 ms. At 10 ms per scan the input is true for 2000 / 10 = 200 scans, give or take one, depending on where in the scan the input is sampled. A level-triggered toggle flips the state about 200 times during the hold, one flip every 10 ms. A 200-flip count is even, so the state ends where it started, in run. Sample so the count lands on 201 and the machine ends paused, by accident. Either way the pause flickered 200 times and the final state is a sampling accident.
Result: with edge detection the same 2 s press produces exactly one change, run to pause, and it stays. Decision: every toggle on a button an operator can hold must be edge-triggered, and the threshold is your own scan time. At a 5 ms scan the double flip starts above 5 ms of hold; at a 20 ms scan above 20 ms. Real held presses run from hundreds of milliseconds to seconds, tens of times above either threshold, so a level-triggered pause toggle works only by luck of the count.
The fix: one shared pause state and a separate light
- Keep every pause button in momentary mode.
- Filter each pause input for 20 to 50 ms. Mechanical pushbutton contacts settle within about 20 ms of touch, so a shorter filter can read contact bounce as extra presses.
- Edge-trigger the press. Copy the filtered input to a last-scan bit, and toggle the retained pause bit only on the scan where the input rises from released to pressed. A held input is true for every scan of the hold, the count explained in how many PLC scans an HMI momentary button bit stays on, and edge detection turns that whole count into one event.
- Point every pause station at the same retained pause bit, so any press makes one change to one state.
- Drive the indicator from that pause bit: a panel-mount indicator light, a stack light tier, or a lamp object on an HMI screen. This is the same state-driven rule behind how PLC-driven HMI graphics represent equipment states.
- Leave the panel's built-in LED alone. Do not wire a controller output into its circuit. The panel drives that LED itself, and probing into the panel puts the panel and your output at risk.
Where the indicator should live is a hardware choice. The annunciator display for two PLCs article compares HMI panel and monitor options for showing machine status next to the line.
Recommendation: Keep the button momentary and treat the panel LED as press feedback only. Keep one retained pause bit in the controller, toggle it once per press with edge detection behind a 20 to 50 ms filter, and light a separate indicator from that bit. Reserve panel toggle mode for the rare machine where a single button owns the state and nothing else ever changes it.
Verify at every pause station
At the first station, press and release once. The machine pauses, the separate light comes on, and the panel LED goes dark on release, which is correct press-feedback behavior. Press again: the machine runs and the light goes off.
Repeat at every other pause station and confirm the same light follows each press, whichever station started it.
Hold a button down for a slow count of five and confirm the state changes exactly once.
Have two stations press at the same time, under controlled conditions. Two valid presses make two changes, pause and then run again. If your machine must ignore a second pause command that close together, write that rule into the logic deliberately so the behavior is chosen, not accidental.
Do this first round of testing with motion stopped or guarded, because you are changing the commands operators use to pause the machine, and run pause and run end to end before production.
Questions
Can I wire my controller output to the panel's built-in LED?
No. The LED is fed by the panel's own driver and the panel offers no documented way to command it from a program. An output wired into that circuit fights the panel's driver and can damage the panel, the output or both. Fit a separate indicator and drive it from the retained pause state.
Does toggle mode on the panel hold the LED on?
In toggle mode the LED follows the switch position, so it holds between presses, and the contact holds the state too. That matches the machine only while one button owns the state exclusively. The first extra pause station, an auto-resume or a guard circuit that changes the state leaves the LED disagreeing with the machine, which is why momentary plus a state-driven light is the durable arrangement.
Do I need a pause light at every station?
No. One indicator driven from the shared pause bit is enough, and it is the correct arrangement: it shows the machine's real state no matter which station paused it or what resumed it. Extra lights at other stations are operator comfort if you want them, and each should be driven from the same state bit so they can never disagree.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.