The .LOG files on the USB drive are the panel software's own trend format, so no editor, rename or import setting turns them into a spreadsheet. Tick Save Log Data on the trend object's Options tab in the HMI project, download it, and the files the panel writes after that open in Excel as CSV.
You pull the USB drive out of the panel, open the trend folder, and every file ends in .LOG. Notepad shows symbol soup, Excel shows one column of garbage, and the manuals say nothing about the format. The files are fine. The trend object on the screen is writing its history in the HMI software's own compact format, and no editor, rename or import setting will read it. The fix lives in the project, not in the files: tick Save Log Data on the trend object's Options tab, download the updated project, and every file the panel writes from then on is CSV that Excel opens directly. This page covers why the .LOG files look scrambled, the exact setting to change, how much drive space the CSV output needs, and how to verify the first export before it feeds a report.
Why does the trend log file open as random characters?
The .LOG file on the USB drive is the trend object's own historical record, written in the HMI software's compact internal format with no commas, tabs or line structure, so Notepad, WordPad and Excel all show random characters because there are no text fields there to read.
What each symptom tells you:
- The same symbol soup in every editor: the file holds the trend's packed samples, and it is intact.
- An Excel delimiter import returns one wide column of junk: there are no separators in the file, so no import setting can find fields that are not there.
- File sizes grow day to day while the content never looks like text: the object is logging correctly, just not as text.
- Renaming to .CSV changes nothing on screen: the name is a label, and the bytes are untouched.
None of this is corruption. The .LOG file is doing the historical job the project gave it, storing the samples behind the trend graph in a format specific to the software. That is also why the manuals are quiet: the format has no published reader for a spreadsheet. The output format is decided in the project, so that is where the change goes.
Can I fix a .LOG file by renaming it to .CSV?
No: renaming edits the name on the directory entry and nothing else, so the bytes inside stay identical and Excel shows the same random characters after the rename as it did before.
The extension is a label the operating system uses to pick a program. It carries no information about the bytes, so no rename can turn a packed record into comma-separated text. The same logic rules out the other workarounds people try first:
- Import delimiters in Excel only help text that already contains separators. This file has none.
- A hex view or a second editor tells you what you already know: there is no hidden text block waiting to be extracted.
- Converters advertised for any LOG file cannot know this format. Run none of them against the only copy of your history.
Keep the originals in place, work on copies for any experiment, and spend the effort on the project, which is the next section.
How do I make the panel write CSV trend files instead?
Open the HMI project in its own programming software, select the Line Trend Graph object that records the signal you report on, tick the check box labeled Save Log Data on its Options tab, save the project, and download it to the panel.
- Get the current project. Load the backup that matches what the panel runs, or pull the project from the panel the way your site normally does. Editing a copy of the wrong project sends you in circles.
- Find the trend object. In this kind of project it is the Line Trend Graph object on the screen that records the process values.
- Open its Options tab and tick Save Log Data. That check box is the switch between the internal trend record and CSV files on the USB drive.
- Save the project under a new name and keep the pre-change file, so the previous configuration can go back if the download causes a problem.
- Download to the panel in a planned stop or another window when operators can spare the screen, following your site's normal change procedure.
- Repeat steps 2 to 5 for each trend object that needs spreadsheet output.
Recommendation: tick Save Log Data on every trend object that feeds a report or a quality record, download the updated project at the next scheduled stop, and check one full shift of output before the data is used for anything that matters.
Which trend objects need the setting changed?
Every trend object that records a signal anyone reports on needs its own tick, because Save Log Data is a per-object setting: changing one Line Trend Graph leaves every other trend object writing .LOG exactly as before.
List the screens in the project, note which objects write to the USB drive, and tick each one. While the object is open, look at the sample interval too, because it decides both the detail and the file size. A 5-second sample is a common choice for line speeds and temperatures; a slower process can sample every 30 or 60 seconds and lose nothing. Pick the interval from how fast the value truly changes, not from how fast the screen can redraw. A graph that spikes while the reading holds steady is the classic sign of an interval mismatch; why an HMI trend graph spikes while the number stays stable explains that symptom.
Worked example: how much USB space do the CSV files need?
Size the drive from rows and bytes, not from the old files: four pens sampled every 5 seconds over two 8-hour shifts produce 11,520 rows per day, about 345,600 bytes at roughly 30 bytes per row, under 8 MB for a 22-day month.
| Step | Numbers | Result |
|---|---|---|
| Rows per shift | 8 h x 3600 s / 5 s | 5,760 rows |
| Rows per day | 5,760 x 2 shifts | 11,520 rows |
| Bytes per day | 11,520 x 30 bytes | 345,600 bytes, about 338 KB |
| Bytes per month | 345,600 x 22 production days | 7,603,200 bytes, about 7.3 MB |
| Bytes per year | 7,603,200 x 12 months | about 91 MB |
The 30 bytes per row assumes a timestamp plus four numeric values with separators; that is an estimate, so measure your first real export and substitute its number. Even with wider timestamps, a year of this logging fits on a small drive. Two more things decide how many files you end up with: the software starts a new file on its own schedule, and how an HMI trend log file rolls over at midnight on USB walks through that timing. Leave headroom on the drive and check free space monthly.
How do I check that the new CSV file is readable?
Let the panel log one normal shift after the download, pull the file from the drive, and check three things: a .CSV name, a first line of readable column names, and a row rate that matches the sample interval, 12 rows per minute at 5 seconds.
- Copy the file to a PC and open the copy, so the original stays untouched.
- Open the copy in a plain text editor first. Names separated by commas in line one mean the format changed; symbol soup means the project running on the panel is still the old one.
- Check the row rate: at a 5-second sample, 60 / 5 = 12 rows per minute, so ten minutes of running should give about 120 rows.
- Spot-check values: pick two or three timestamps and compare the CSV numbers with what the trend display shows for the same times.
- If line one carries no names, fix the template once instead of cleaning every export by hand; adding column headers to an HMI trend log file on USB shows how.
Why is there no CSV file, or no rows, after the download?
Check three causes in this order: the updated project is not the one running on the panel, the tick went into a different trend object than the one logging, or the USB drive is absent, full or unreliable, and each leaves a different fingerprint.
- No new file at all, and the trend still draws: the download did not take, or the box was ticked in a project copy that never reached the panel. Confirm the project version on the panel and repeat the download.
- A new file appears with the old .LOG extension: the object still logging is not the one you changed. Go back through the project's screens and tick every trend object that writes to the drive.
- A .CSV appears but rows stop after a while: look at free space first. When the drive fills, logging stops or old files disappear depending on the rollover setting; confirming oldest-file rollover when the USB log drive fills is the check to run.
- A gap inside the CSV: no rows exist for a period when the panel was powered down or the project was not running. The logger writes nothing while the panel is off, so the missing stretch matches the downtime.
What happens to the old .LOG files already on the drive?
The tick changes what the panel writes next and leaves every existing .LOG file exactly as it is, so archive the originals untouched, label them so the next person can find them, and treat converting them as a separate job.
Move the old files to a dated folder, or rename the copies with a pattern the whole team follows; renaming data log files by job number shows one such pattern. If someone needs the history inside a .LOG file, use the reader or converter published for the exact software version on the panel, and run it on copies. Where no reader is documented, send the software vendor's support a copied sample file plus the version number and ask for the supported path; guessing with converters is how trend history gets lost.
Questions
Do I need special software to read the new CSV files?
No. CSV is plain text with commas between the fields, so Excel, any spreadsheet, most text editors and SQL import tools read it without a converter. That is the point of the switch: the same samples that were locked in the .LOG format arrive as rows and columns you can sort, chart and paste into a report or a quality record. Nothing on the panel side changes for operators.
What if the project file for the panel is lost?
Search first: the PCs that touch the panel, the network folder your site keeps for panel backups, and any copy from the last service visit. The setting lives in the project, not on the panel, so nothing changes until a matching project is open, ticked and downloaded. If no project turns up, ask the software vendor's support about recovering one for your panel, and expect the unit to keep writing .LOG until then.
Does the sample interval change what the CSV contains?
It changes the row count, not the values. At a 5-second sample the object writes one row per pen every 5 seconds; cutting the interval to 2.5 seconds roughly doubles the rows and the file size while adding little about a slow process. The values themselves are the pens configured on the trend object, so the interval never adds a signal that was not on the graph.
Planning information, not legal or engineering advice. Rules differ by place: check codes and permits with your local authority and your licensed trades.