Paging back through an HMI trend parks the view on stored samples, so the trace sits still and the web page looks frozen with it. Read the timestamp at the trend's right edge, step the chart forward to the current time, confirm two new samples at the configured interval, then reload the browser page. Edit the trend project only when the source value moves and the chart still does not.
An HMI trend that stops moving right after you page back through history is almost never a broken instrument. The view is parked on stored samples, so the trace sits still until its right edge comes back to the newest one. The web page can look frozen for a separate reason, because a browser only shows what the page last fetched. Read the timestamp at the trend's right edge and compare it with the panel clock: within a few seconds you will know whether this is a parked view, a static source value or a refresh problem. The steps below take you through all three, with a worked cold-store example.
What paging back changes
The vendor's help file for a typical line trend object describes it as a digital chart recorder: the object watches its assigned tag and draws the samples as they arrive, for as long as the screen runs. The scroll arrows are a browsing tool. They choose which stored samples the window draws; they do not switch the sampling off.
Two documented details help you read the screen. Some trend controls can draw from the live process or from a stored log, and the control that picks between the two is named in the panel manual. And on at least one panel family, history mode announces itself: pressing the "M" button puts the trend into scroll browsing and an on-screen scroll arrow appears. If your chart shows that scroll arrow, the view has left the live edge. Look for the cue before you suspect the tag.
Symptoms and what each points to
- The right-edge timestamp is behind the panel clock. The view is parked in history. Fix this first, with the steps in the next section.
- The right edge reads the current time, the trace is flat and a numeric display of the same tag is flat too. The value itself is static: sensor, PLC value or comms. The chart has nothing new to draw.
- The numeric display changes but the trace stays flat. The chart is the suspect: check the trend's assigned tag and its sampling and history settings in the project.
- The chart on the panel moves but the browser page does not. That is page refresh behavior, covered further down.
- Nothing on the panel moves, on this screen or any other. That is a frozen panel, not a parked chart: start with the HMI blank and frozen screen checklist, and if a tag value sits still too, the walk-through on finding which layer freezes when a PLC counter stops updating takes the diagnosis back through the layers.
Bring the chart back to the live edge
- Read the timestamp at the right edge of the trend and compare it with the panel clock. Any gap means the view is parked, and the size of the gap tells you how far back you scrolled.
- Step forward with the trend's forward arrows until the right edge reads the current time. Most trends move one full window per press; note the window width on the time axis so you know how many presses to expect.
- Confirm the history cue is gone: on panels that show the scroll arrow, it disappears once the view is back at the present. Where the project provides a live or real-time control, that is the faster route when the arrows have a long way to travel.
- Wait two sample intervals at the rate configured in the trend and confirm two new points at the right edge. Read the rate from the trend configuration; do not infer it from how often the screen redraws.
- If the forward arrows will not reach the present, put the editor away and look for a control labeled live, real-time or auto on or around the trend object. Labels differ between models and software revisions, so use the ones on the installed project.
Check the source value before touching the project
A chart cannot draw movement out of a value that is not changing. With the chart back at the live edge, compare the trace with a numeric readout of the same tag on the panel. Number moves, trace moves: monitoring is restored and the incident is closed. Number static: the fault is upstream of the screen, in the sensor, the PLC value or the comms path, and trend settings are the wrong place to look. If you do open the project, write down the tag assignment and the trend settings first so the working configuration can be put back.
Example: cold store trend, 1 second samples
Here is the whole check on real numbers. The trend samples the store temperature every 1 s, the window shows 5 min of history and the store setpoint is -18 °C. How much history a window holds is a project choice; the piece on showing the full test duration on a live trend chart covers sizing it on purpose.
At 14:15:00 the operator reports the chart frozen. The right edge reads 14:05:00, so the view sits 10 min, or 600 s, behind the clock: parked, not broken. The trend steps one window per press, 5 min = 300 s, so two presses bring the right edge to 14:15:00. Two sample intervals later, at 14:15:02, two new points have appeared and the trace is extending at 1 sample per second. The numeric readout beside the chart holds -18.2 °C and drifts a few tenths as the compressor cycles, which agrees with the trace. The browser page still shows the 14:05 picture until someone reloads it. Total fix: two button presses and a page reload, with no project edit and no lost monitoring time.
The web page is a separate check
A browser page shows only what it fetched when it loaded. Unless the page reloads itself, the picture sits at that moment even while the panel chart advances normally. So judge the panel first and the page second.
Watch the chart on the panel. When it moves again, reload the browser page and watch what happens next. If it updates on its own afterward, page refresh works and the parked chart was what made every refresh look identical. If it only changes when you reload, the page has no self-refresh switched on, and whether one can be enabled is a project setting the panel's web-server documentation names. Either way, write down which behavior remote operators get; a page that needs a manual reload is fine as long as everyone knows it. Pressing the panel's back arrows repeatedly will not freshen a stale page, and browser refresh will not drag a parked chart forward: they are two separate checks. Remote viewer pages add quirks of their own on top of this; this knowledge-base page on HTML viewer scrolling and tooltip issues records similar viewer behavior on another system.
The decision
Recommendation: Check the trend's right-edge timestamp against the panel clock before you edit any trend setting. If it reads even one sample behind, step the chart forward to the present, confirm samples arriving at the configured interval, then reload the web page and note whether it refreshes on its own. Open the project only when three things are true at once: the right edge sits at the current time, a numeric readout of the same tag changes, and the trace still holds flat. Then check the trend's tag assignment and its sampling and history settings. Save a copy of the working project before you change anything, and if this trend is the store's temperature record, make the edit with the responsible controls technician and keep an independent record of the store temperature while you work.
Questions
Does the panel keep logging temperatures while the chart is parked?
The scroll arrows choose what the window draws; the sampling itself is a separate job the trend object keeps doing, which is why the fix is to move the view forward rather than restart anything. If the project also writes temperatures to a data log, that record is independent of the chart: check its timestamps and you will see the gap on screen is only a gap in the drawing. A chart and a log are configured separately, so your project manual tells you whether a log exists and how far back it reaches.
How do I stop operators parking the trend in history?
Put a numeric readout of the same tag next to the chart. A number that keeps changing tells a parked view from a real freeze without opening anything. Some projects also add a screen or button that returns the trend to the live view; whether you can offer operators a one-touch return depends on the panel software, so check the trend object's controls in your editor before promising it.
Why does the web page stay frozen even after the chart moves again?
A browser shows only what the page fetches. If the project's web page has no refresh of its own, the picture stays at the moment it loaded even with the panel chart advancing normally. Reload the page and watch: if it only changes when you reload, tell remote operators that reload is the update method, or ask whoever owns the project whether a refresh can be switched on for that page.
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