Why so many formats?
The same test can produce very different files depending on the tool that records it. Automotive data loggers write MDF4, National Instruments benches write TDMS, cyclers often export CSV or Excel, and field tests produce GPS tracks. The trouble starts when these files travel: to a customer, a supplier or a colleague who does not have the original software.
Formats at a glance
| Format | Extension | Type | Origin | Typical use |
|---|---|---|---|---|
| MDF4 | .mf4 | Binary | ASAM standard | Vehicle measurements, CAN bus, data loggers |
| NI TDMS | .tdms | Binary | National Instruments | Test benches running LabVIEW, VeriStand, DIAdem |
| CSV | .csv, .txt | Text | Universal | Exports from cyclers, loggers and software |
| Excel | .xlsx, .xls | Spreadsheet | Microsoft | Sharing results, cycler exports |
| GPX | .gpx | XML | Open standard | GPS tracks, road tests |
| FIT · TCX | .fit, .tcx | Binary · XML | Garmin | Connected sensors, field tests, light mobility |
MDF4 (ASAM MDF)
MDF, for Measurement Data Format, is standardised by ASAM and widely used in automotive. Version 4 (.mf4 files) records many signals, often from CAN buses and on-board sensors, organised in channel groups sampled at different rates. The file can also hold conversion rules from raw values to physical quantities.
Common pitfall: two signals in the same file do not necessarily share a time base. To compare them, resample them onto a common time axis.
NI TDMS
TDMS is National Instruments' format, written by LabVIEW, VeriStand, DIAdem or FlexLogger. It organises data on three levels: file, groups and channels, each with its own properties (unit, sample rate, start time…). It often comes with a .tdms_index file, which only speeds up reading.
Common pitfall: opening or sending the .tdms_index file instead of the .tdms. The .tdms file is the one that holds the measurements.
CSV
CSV is a text file where each row is a point in time and each column a measurement channel. It can be read anywhere, which makes it the most common exchange format.
Common pitfalls: the separator (comma or semicolon), decimal commas in files produced on European systems, character encoding and date formats. On long tests, files also become very large.
Excel
The .xlsx format is handy for sharing results, and many cyclers export their data to Excel, often across several sheets.
Common pitfall: an Excel sheet is limited to 1,048,576 rows. A long, finely sampled test quickly exceeds that limit, and the export is then split or truncated.
GPX, FIT and TCX
These formats come from GPS tracks and connected sensors. GPX is an open XML format for position, altitude and time. FIT, a compact binary format, and TCX, an XML format, were created by Garmin and add data such as speed, power, cadence or heart rate.
They are useful for road tests, validation drives, e-bikes and light mobility, where a measurement needs to be tied to a position and a time.
How to open these files without a licence
Thanks to NeydFlow, a free viewer is available to open and view your MDF4, NI TDMS, CSV, Excel, GPX, FIT and TCX files right in your browser, with no software to install. It comes with the free NeydFlow account.
If you work in Python, open-source libraries also exist, such as asammdf for MDF and npTDMS for TDMS.
When one file is no longer enough
Comparing dozens of tests, applying the same calculations to a whole campaign or sharing results with a team takes more than a viewer. That is what test data post-processing with the NeydFlow platform is for.