How to diagnose USB dropouts and read-rate collapse

Are intermittent dropouts really random?

A fault that comes and goes feels random, but it usually is not. Four causes account for nearly all such faults, and each one has a timing signature. Before you change anything, spend one session just watching. Note the clock time of each drop, what you were doing and how long the machine had been running. That one note will save you three evenings of swapping parts.

Which cause matches your symptom?

Match the shape of your failure rather than its wording, then use the right-hand column, which gives the cheapest test that tells that cause apart.

Two causes can stack, and a marginal cable on a shared controller fails in a way that looks like neither cause on its own.

What is USB selective suspend?

Windows is allowed to power down a USB device that looks idle, and USB 3 links have their own low-power states on top of that. For a mouse this makes no visible difference, but a bridge that is about to be asked for a sustained stream wakes up slowly. It wakes slowly enough that the software sees a failed transfer and gives up. Two switches need clearing: the checkbox on the device's Power Management tab in Device Manager, and USB selective suspend in the active power plan. Clear both on the second PC and retest before you suspect anything physical.

Do other USB devices share the card's controller?

A motherboard's rear panel may show eight USB ports but route them through two or three controllers. Anything on the same controller as your card shares its bandwidth and interrupt path, so a capture card two sockets away can make reads stutter. To see the real tree, switch the Device Manager view to Devices by connection. A free USB tree viewer utility shows the same thing more clearly. Put the card on a controller of its own and leave it there.

Could the USB cable be the problem?

USB-C cables are not interchangeable, because many are wired for charging only and have no SuperSpeed pairs at all. Plenty of cheap data cables also cannot hold a USB 3 link over a long run. A socket that has been pulled sideways for six months will drop out under vibration. Keep one short cable that you know is good as your reference, and test with it whenever a fault appears. If switching to it changes the behaviour, stop investigating everything else.

Can heat make the card drop out?

An FPGA and a USB 3 bridge both dissipate real power. A low-profile board pressed against a hot GPU, in a case with no side intake, has nowhere to put that heat. The sign is timing: the failure arrives after roughly the same interval every session and clears once the machine has been off for a while. We do not quote a temperature figure, because it varies by board and none of the numbers people post were measured on yours. Test the theory instead by opening the case, aiming a fan at the board and checking whether the interval changes.

How to capture evidence of a dropout

These read-only commands run on the SECOND PC. Run them before a session and again straight after a drop, and keep both outputs, because the difference between them is the evidence.

REM Which USB controllers exist, so you can see what shares what
powershell -c "Get-CimInstance Win32_USBController | Select-Object Name, DeviceID"

REM Every USB device and its current status. Look for Error or Unknown
powershell -c "Get-PnpDevice -Class USB | Select-Object Status, Class, FriendlyName, InstanceId"

REM Is the FTDI bridge still present right now?
powershell -c "Get-PnpDevice | Where-Object FriendlyName -like '*FT601*' | Format-List"

REM Recent device-installation and PnP configuration events
powershell -c "Get-WinEvent -LogName Microsoft-Windows-Kernel-PnP/Configuration -MaxEvents 40 | Format-Table TimeCreated, Id, Message -Auto"

REM Surprise-removal and driver-load failures in the System log
powershell -c "Get-WinEvent -FilterHashtable @{LogName='System'; StartTime=(Get-Date).AddHours(-6)} -MaxEvents 60 | Where-Object ProviderName -match 'USB|PnP|Disk' | Format-Table TimeCreated, Id, ProviderName"

REM Which sleep states this machine supports, which tells you whether suspend can be in play
powercfg /a

How to keep a dropout log a supplier can use

Three sessions of this log are worth more than three pages of description, because the log turns a ticket from a conversation into a fix.

1. 01Write down the start time of the session and the machine's uptime
2. 02Note the read-speed figure at the start of the session
3. 03When it drops, note the clock time and what was happening: idle, loading, a scene change, or another device in use
4. 04Copy the exact error string the software printed, as text
5. 05Run the status and event commands and save their output next to the timestamp
6. 06Change exactly one variable, such as the cable, the port, the power setting or the airflow, and repeat the session
7. 07Send the log with that one change named, so whoever reads it can see cause and effect rather than a list of everything you own

What to rule out, in order

The list starts with the cheapest and most common causes, and most dropout tickets end at item three.

• USB selective suspend is off, in both the per-device checkbox and the power plan setting
• The card is plugged directly into a rear USB 3 port, with every hub, dock and extension removed
• One short known-good data cable is the reference for every later test
• The card is alone on its USB controller, with busy devices moved elsewhere
• A memory map is in use, so the read path is never walked into unmapped ranges
• The card has airflow over it, tested with the side panel off before anything is bought
• Both machines are cold-booted properly between tests, not restarted

Common questions about USB dropouts