The visible span of an HMI trend is plotted samples times sample interval, so decide the hours first, then set the interval and the trend's point capacity to match. Two pens, solid for the measured temperature and dashed for the setpoint, put both curves on one screen without scrolling.
A trend fits a whole production run on one screen when its visible time span covers the run, and that span is the plotted sample count times the sample interval. Set the span first, pick the interval second, size the trend third. Scrolling only moves through recorded history; it never widens the window. The setup below puts a solid trace for measured temperature and a dashed trace for setpoint on one panel screen for a 10-hour batch, with the arithmetic that sizes both.
What decides the visible span
Two settings decide how much time the panel shows before scrolling starts:
visible span = plotted points × sample interval
Ten hours at one sample per minute needs 600 points per pen. The same ten hours at one sample per 15 minutes needs 40 points. Most complaints about short history come from the default rate: at least one major HMI family documents its trend default at 1000 ms, one sample every second, and at that rate ten hours of history requires 36,000 samples per pen, which exhausts any point budget fast. Compute the interval from the process, then enter it where the trend's sample-rate setting lives, in the trend object's properties.
The chart has a redraw limit of its own. When the interval you compute is shorter than the trend can update, the trace stalls; why an HMI trend graph will not update every half second walks through that limit.
What you need
- A trend object with at least two pens, in a panel project you can edit.
- A PLC address or tag for measured temperature and one for setpoint. If the temperature is a REAL held across two word registers, the pen's tag points at the pair; displaying a PLC float stored in two word registers on an HMI shows the register pairing.
- The trend object's properties, where the sample-rate setting and the pen list sit; on many panels the sample-rate control sits at the bottom of that window.
- The cycle length and the fastest event operators must catch, both in minutes.
Set it up
- Fix the span. Write down the run length the screen must show, for example 10 h for one oven batch.
- Pick the interval from the process. An event shorter than about two sampling intervals can begin and end between two samples and never reach the plot, so a fault that can clear within 20 minutes needs an interval of 10 minutes or less.
- Compute the points: span divided by interval, per pen. Two pens store twice that.
- Size the trend to hold at least that point count. If the object caps points, lengthen the interval until points × interval still covers the span.
- Enter the interval in the trend's sample-rate setting and set the history length or span the object asks for.
- Map the pens. Assign the measured-temperature tag to one pen and the setpoint tag to the other; draw the measured value solid and the setpoint dashed, in colors that stay distinct on a shop floor screen.
- Run the project and go to the checks below.
Keep the over-temperature trip in the PLC: protection never depends on screen refresh.
Example: a 10-hour oven batch on one screen
An oven cures a load for 10 hours at a 180 °C setpoint. The operator must see the ramp, the soak and any overshoot lasting a few minutes, so the interval is set to 5 minutes:
600 min ÷ 5 min = 120 samples per pen
Two pens store 240 values in total, and 120 points × 5 min = 600 min, the full batch. Set the sample rate to 5 minutes and give the trend room for at least 120 points per pen; the whole run then sits on screen with no scrolling.
| Sample interval | Points per pen for 10 h | What the trace shows |
|---|---|---|
| 1 h | 10 | the shape of a slow shift, nothing more |
| 15 min | 40 | slow drift; a fan fault that clears within 30 min can fall between samples |
| 5 min | 120 | ramp, soak, overshoot and recovery |
| 1 min | 600 | fine detail at five times the 5-minute point budget |
The decision for this oven: interval 5 min, capacity 120 points per pen, one solid pen and one dashed pen mapped to their registers.
How to check it worked
- Step the setpoint from the panel or the PLC and confirm the dashed pen moves with it.
- Compare the solid pen with a trusted sensor readout of the same process. If the trace spikes while the number stays steady, sample timing is the suspect; why an HMI trend graph spikes while the number stays stable explains that mismatch.
- Watch one change land on the plot: at a 5 minute interval, a temperature change just after one sample must appear at the next sample, 5 minutes later.
- Confirm the span: once the buffer holds its 120 points, the window covers 600 minutes and the batch start stays visible without scrolling.
If it still scrolls
- The interval is too short for the point budget: recompute points = span ÷ interval and raise the interval until the product fits the buffer.
- The trend object is too small for the point count: enlarge the plotting area, or accept a shorter span with the faster interval.
- The sample rate is still at default: 1000 ms produces 3,600 samples per pen each hour, so a default trend fills its history in minutes. Reset the interval from the table above.
Recommendation
Recommendation: Set the span first and derive the rest. For a 10-hour batch, sample every 5 minutes, give the trend at least 120 points per pen, and map a solid pen to the measured temperature and a dashed pen to the setpoint. Move to 15 minutes only when the slowest event operators must catch lasts at least 30 minutes, and to 1 minute only when the point budget carries 600 points per pen. Recompute both numbers whenever the cycle length changes.
Questions
What if the trend object limits how many points it keeps?
Read the point limit in the trend object's properties, multiply it by the sample interval, and compare that product with the span the run needs. If the product falls short, the interval is the only setting that stretches the span, so lengthen it until points times interval covers the run. Keep the interval short enough that the shortest event operators must catch lasts longer than about two intervals.
Why does a fresh trend show only a few minutes of history?
A trend buffer fills from the moment the project starts running, so the window grows over time and no data exists before that moment. At the 1000 ms default many panels use, the buffer holds its first full hour after 3,600 samples per pen; at 5 minutes per sample it fills in 10 hours. Mark the project start time, wait for points times interval to elapse, and the full span appears without scrolling.
Does the trend's sample rate load the PLC link?
The panel requests each pen's value once per interval. Two pens at a 5 minute interval make 24 requests per hour; the same two pens at the 1000 ms default make about 7,200. On a serial link that difference decides whether the rest of the panel stays responsive. The PLC scan is unaffected; the trend only reads the registers.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.