Short answer: same thing. DUT (device under test) and UUT (unit under test) both mean the piece of hardware sitting in the fixture right now, about to find out what it's really worth. Test engineers have used both for decades, often in the same meeting, sometimes in the same sentence.
So why two names? Mostly geography. Not countries, industries.
Who Says What
| Term | Stands for | Where you'll hear it |
|---|---|---|
| DUT | Device Under Test | Semiconductors, RF, lab benches, instrument manuals, OpenHTF |
| UUT | Unit Under Test | Aerospace, defense, MRO, NI TestStand, IEEE 1671 (ATML) |
A few ways to tell which camp you're in:
- Someone mentions the DUT board on a semiconductor tester: DUT camp.
- Your test executive has an entry point called Test UUTs: you're in TestStand country.
- OpenHTF, which started at Google, stores the ID of whatever it tested as
dut_id. NI and Google never settled this, and the hardware never noticed.
Is There Any Real Difference?
Officially, no. Unofficially, some teams use a size rule:
- DUT for something small: a chip, a sensor, a bare board
- UUT for something you'd rather not drop on your foot: an assembly, a line-replaceable unit, a finished product
That's a house convention, not a standard. If your team has one, put it in the test plan. If it doesn't, please don't open a Slack thread about it. (Someone already has.)
The Rest of the Acronym Zoo
Once you spot the "under test" pattern, it's everywhere:
| Term | Stands for | Habitat |
|---|---|---|
| EUT | Equipment Under Test | EMC and safety labs (IEC 61000-4, CISPR) |
| SUT | System Under Test | System-level and software testing |
| DUV | Design Under Verification | Chip verification, before there's any silicon |
| IUT | Implementation Under Test | Protocol conformance testing |
| BUT | Board Under Test | Nowhere official. Yet. |
And if you work with French teams: DUT is also a French two-year technology diploma, held by plenty of test technicians. It was replaced in 2021 by the BUT. So in France, a DUT really can test a DUT.
What Actually Matters: the Serial Number
No yield has ever dropped because of an acronym. What does break is identity: the same board recorded under two IDs, or a repaired unit that comes back from rework as a stranger.
Whatever you call it, the thing under test needs three fields:
| Field | Why it matters |
|---|---|
| Serial number | Ties every run, retest and repair to one physical unit |
| Part number | Groups units of the same design for yield and Cpk |
| Revision | Keeps REV-A data out of REV-B statistics |
In TofuPilot we skipped the debate and just called it a unit:
unit: serial_number: default_value: "SN-000001" part_number: default_value: "PCBA-100"Every run attaches to a unit by serial number, so its full history lives in one place, whatever your team calls it. If you're coming from OpenHTF, the dut_id you return from test_start becomes the unit's serial number.
Common Mistakes
| Mistake | Consequence | Fix |
|---|---|---|
| Debating DUT vs UUT in a design review | One lost afternoon | Pick one, write it in the test plan |
dut_sn on one station, uut_sn on another | Two columns that never join | One identity field across every station |
| Testing before the serial is known | Retests look like new units, FPY lies | Scan or read the serial before the first phase |
| New serial after rework | Repair history split across two "units" | Keep the serial, record the rework as a run |
FAQ
Is DUT or UUT more correct? Neither. Use whatever your industry, your customer or your test executive already uses.
Can I use both in the same document? You can. Your reviewer will circle it in red. Pick one per document.
What do I call it in TofuPilot? A unit, identified by its serial number. That's the whole naming scheme.
