To the PC, a USB-serial adapter (USB to RS-485 or RS-232) is a USB device. When the USB link drops, Windows removes the COM port device and creates it again, and any program that had the port open gets «device not found» or write errors. This guide is about that case — the connection itself going away. If the link stays up but replies are late or frames split, that is the latency timer (see the USB-serial latency guide); if there is simply no reply, it is usually wiring or settings (see the Modbus no-reply guide).
Sort it by symptom first
| Symptom | Look at first |
|---|---|
| The port briefly disappears from Device Manager and comes back | The USB link dropped and re-attached — steps 1–4 below |
| The COM number changes when replugged | The adapter has no serial number, or a different USB port was used — section 5 |
| Only the first send after a long idle fails | Power saving — section 3 |
| The port stays but shows a warning icon in Device Manager | Driver and problem code — section 4 |
| The port stays, only some replies go missing | Not a disconnect — check latency timer, timeouts and wiring |
Confirm it really disconnected by looking at the record. The Windows event log (Kernel-PnP) logs every time the device starts, so a day with several restarts means the link dropped and came back. The «History» card in PortDock Port Inspector collects these events per port and flags any day with three or more reconnects as «Look at».
1. Cable and connector — does the symptom follow?
- Swap the USB cable (remove any extension) and try another USB port. If the symptom follows the part you swapped, that part is the cause.
- An adapter that wobbles or hangs at an angle under the cable's weight can lose contact for an instant. Secure the cable.
- Front-panel ports go through an extra internal cable. When chasing drops, compare with a rear (motherboard) port.
2. Hubs — what sits in between?
- If the adapter goes through a hub, take the hub out and plug straight into the PC to compare.
- A hub without its own power adapter (bus-powered) shares the PC port's power among its devices and can run short with several attached. Use a self-powered hub or fewer devices.
- Port Inspector's «USB path» card shows the chain from the adapter up through hubs and the root hub to the USB controller, each hub's power type (bus or self), the port number and the negotiated speed. Most USB-serial chips attach at Full Speed (12 Mbps), far faster than the serial line, so that is not a slow link.
3. Power saving — does it drop after idling?
Windows can put an idle USB device to sleep port by port («USB selective suspend»). Microsoft's documentation says the feature is on by default and strongly recommends not disabling it, because without it the CPU cannot reach deeper sleep states and power use rises. So rather than switching it off by reflex, first check whether the drops line up with power saving.
- If it only happens «after a quiet spell, on the next use», suspect power saving. If it drops while you are polling continuously, look at cable, hub and power instead.
- Two places to look: Control Panel → Power Options → advanced power settings → USB settings → «USB selective suspend setting», and Device Manager → the adapter (or the USB device and hub above it) → Properties → Power Management → «Allow the computer to turn off this device to save power».
- Consider turning them off on equipment PCs where staying connected matters more than saving power, such as 24/7 monitoring or logging. On a laptop, battery life will drop.
- Port Inspector's «Power saving» card reads both values, on AC and battery, for the adapter and the hubs above it in one go (it does not change them).
4. Driver — the problem code in Device Manager
- A warning icon on the port in Device Manager means there is a problem code under Properties → General → Device status. Microsoft's «Device Manager error messages» explains codes such as 10 (cannot start), 28 (no driver) and 43 (device reported a problem).
- Try the driver from the chip maker (FTDI, Silicon Labs, WCH, Prolific …) before the one Windows Update picked. The device's VID/PID tells you the chip.
- Clone chips use the same VID/PID, and there are known cases where the latest official driver refuses them. See whether the symptom goes away with a different adapter.
5. Why the COM number changes
Windows takes COM numbers from its «COM port database» and keeps a number reserved even after the device is unplugged. That is why a new adapter can come up as COM15 or higher.
If the same adapter gets a new number in a different USB port, it most likely has no serial number (iSerialNumber). As a Microsoft blog post explains, Windows tells such devices apart by their parent — the port or hub they are plugged into — so in another port the adapter becomes a «new device» and gets a new COM number. An adapter with a serial number is recognised as the same device in any port.
- On a site PC, the simplest fix is to always plug the adapter into the same USB socket.
- If a program has the COM number fixed, an adapter with a serial number keeps its number more reliably.
- Numbers held by devices that are no longer present are released by removing the greyed-out ports under Device Manager → View → «Show hidden devices». Editing the reservation in the registry by hand can give two devices the same number, so it is not recommended.
- Some old programs cannot select COM17 or above. Only then lower the number under the device's Properties → Port Settings → Advanced (avoid numbers marked «in use»).
What the program must do after a drop
Once USB drops, the open port is gone; even after replugging, the program has to open the port again. With «Reconnect when plugged back in» on, PortDock Serial opens the same device automatically when it reappears — even if the COM number changed — and logs «RECONNECT COM3 → COM5». If several identical adapters without serial numbers are plugged in, it cannot tell which one is which and keeps waiting; then pick the port and connect by hand.
What the PC cannot see
The checks above are on the PC side. The PC cannot see the RS-485 wiring behind the adapter (A/B swapped, termination, bias), grounding and shielding at the equipment, or noise when large loads switch on. If drops only happen when a particular machine starts, look at its wiring and grounding (see the RS-485 wiring guide).
Sources
- Microsoft Learn — USB Selective Suspend — on by default, Microsoft recommends not disabling it (2025-05-07)
- Microsoft Windows Music Dev blog — The importance of including a unique iSerialNumber in your USB MIDI Devices — without a serial number, another port means a new device (2023-06-06)
- Microsoft Learn — Configuration of COM Ports — COM numbers come from the COM port database
- Microsoft Learn — Device Manager error messages — what the problem codes mean
Published 2026-10-29