A live trend only draws values; the saved record comes from a data log you enable in the HMI, pointed at a memory card or USB stick. Verify the registers first, log the four RTD and analog channels at one-minute intervals, and a 1 GB card holds decades of CSV records.
You can keep RTD and analog values for later analysis, and the job belongs to a data logger, not to the trend graph. The values already exist: an RTD input module writes scaled temperatures into its registers, and an analog input module does the same for its channels. A trend reads those registers and draws lines. A data log writes them out as timestamped records to a memory card or USB stick. This page walks the whole setup: verify each register, name the storage, build the log list, set the interval from the process, prove a file is written, and size the card with arithmetic you can reuse.
What a trend does and what a log does
A trend graph is a display. It polls the registers at its redraw rate and draws. A data log is a separate function in the same configuration software: you list the registers to record, and the runtime writes a timestamped line per sample to storage. Most HMI configuration packages keep the log list with the tags and put the destination in a runtime storage setting where you pick the SD card slot or a USB stick. Some controllers log without any screen at all; one major PLC maker documents data logs written as CSV files into the controller's own persistent flash memory, readable through its built-in web server.
Point the log at removable storage. The panel's internal memory is a fixed space that leaves with the panel at replacement, while a card costs little and pulls out for the analysis step. What a panel can hold internally is its own question; see whether an HMI panel can store PLC data for later retrieval.
What you need
- The PLC program and the input module documentation, which lists the registers each RTD and analog module occupies and the format of the stored value.
- An HMI panel or a standalone logger with a memory card or USB slot, plus its configuration software.
- A card or stick for the logs. The worked example below shows 1 GB already runs years at minute-scale logging.
- A computer with a spreadsheet for the CSV files.
- A way to make one known change: warm water, a second thermometer and, for an analog input, a loop calibrator.
Operators at the machine may also want to watch the values live. That job belongs to a separate notice screen, not to the logging panel: we build screens for it, and the display range page lists the sizes.
Set up the log
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Verify each register before logging it. Watch the live value at the address while you make a real change at the sensor: warm water on the RTD bulb, or a loop calibrator on the analog input. Create the change at the sensor, with the process in a safe state, and leave PLC forcing out of the test. Compare the number against the module documentation, because raw counts, scaled integers and implied decimals store the value differently, and a register read in the wrong format shows plausible nonsense; the article on nonsense numbers from a PLC analog register shows the pattern. Write down the address, format, scale and units for every channel.
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Name the storage destination. In the HMI configuration software, open the runtime storage setting and point it at the card slot or USB port. Choose the slot and the card deliberately; the page on enabling HMI data logging to a CF card and picking the safe slot covers that choice.
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Build the log list. Enter one logging tag per channel and name the columns so a file from last year still explains itself, such as Pump1_Winding_C; the page on adding column headers to an HMI trend log file on USB shows how. Give each log its own file name so one run never overwrites another; see giving each HMI data log a unique CSV file name.
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Set the interval from the process. Pump windings and process lines move over minutes, because the metal mass sets the pace, so a 60 s interval records everything you can act on. Use a 1 s interval only across the short window of a trip, a start or a valve stroke. Mind the trend too: a graph that redraws faster than its logger writes shows points the file will never contain, which is the subject of why an HMI trend chart records fewer samples than the trigger.
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Enable logging and run a controlled test. Save and download the project, hold the RTD in water near 50 °C, and watch the live value climb. Let it log for 15 minutes, then pull the card and open today's file on a computer.
How to check it worked
- The file exists on the card with the date in its name. Rollover rules vary by panel; the common midnight pattern is covered in how an HMI trend log file rolls over at midnight on USB.
- The file opens in the spreadsheet with no conversion step. A file in the panel's own trend format is not an export; exporting CSV when the HMI trend LOG file is not readable is the fix.
- All channels appear, and the timestamps advance by exactly the interval: 12:00:00, 12:01:00, 12:02:00.
- The warm-water ramp shows in the RTD column, at the minute it happened.
Example: four pump channels on a 1 GB card
Scenario: a pump skid with three winding RTDs and one 4-20 mA pressure sensor, all logged at a 60 s interval. Each CSV record carries a timestamp and four values. In the common yyyy-mm-dd hh:mm:ss layout the timestamp is 19 characters, four values run about 5 characters each, and four commas plus the line ending add a few more: about 45 bytes, call it 50 with the header spread over the file.
At one record per minute, a day holds 24 × 60 = 1,440 records, and 1,440 × 50 = 72,000 bytes, about 72 kB. A 1 GB card holds 1,000,000,000 bytes (card makers print decimal gigabytes), so 1,000,000,000 ÷ 72,000 ≈ 13,900 days, about 38 years. Even a 64 MB stick gives 64,000,000 ÷ 72,000 ≈ 890 days, about 2.4 years.
At a 1 s interval the same record holds 86,400 samples per day and 86,400 × 50 = 4,320,000 bytes, about 4.3 MB, so the card runs 1,000,000,000 ÷ 4,320,000 ≈ 231 days, about 7 months.
The decision falls out plainly: at minute rates the card size never decides anything, so set the interval from the process and size the card to how often you want to pull it. Keep one formula: bytes per day = records per day × bytes per record.
When the PLC logs by itself
Several current controllers include a data log function, so a machine with no screen can still keep a record. One major PLC maker documents logs written as CSV into the controller's persistent flash, read back through its built-in web server. The limits are space and reach: the flash partition is fixed, and the files come out through a browser or a copy routine instead of a card you pocket. Use it where the cabinet has no screen, or where the record must survive a screen swap, and pull the files on a schedule you keep.
Recommendation
Recommendation: Log the RTD and analog registers from the HMI at a 60 s interval to a memory card or USB stick named in the runtime storage setting, and copy the files off monthly. At 72 kB per day for four channels, a 1 GB card runs for years, so the monthly pull is about backup discipline, not space. Drop to 1 s only across trips and strokes you need in detail, and expect about 231 days per GB at that rate. Where the machine has no screen, use the controller's built-in CSV log and add a screen later when operators need to watch the values.
Questions
Does the logger keep writing if the HMI screen is switched off?
No. The logger runs inside the HMI, so records exist only for the periods the screen is powered. If the log must cover unattended periods, either keep the screen powered through them or use the data log built into the PLC, which writes CSV files to its own flash memory with no screen involved.
The logged temperature disagrees with my handheld meter. Where do I look first?
Check the scale before the sensor. Compare the log against a known condition, an ice bath at 0 °C for an exposed RTD for example, then look at the configured scaling and the register format. A raw count written as degrees, a decimal point shifted one place or a BCD register read as binary all produce files that look plausible and sit wrong by a fixed factor.
Why did my card fill with files I cannot open?
Some panels write a binary trend format instead of CSV. Look for an export function in the runtime or its configuration software that writes CSV, and make the export part of the routine. When you choose new equipment, prefer a logger whose documentation states CSV as the file format, so the analysis step needs no conversion.
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