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On a legacy Windows system that boots with NTLDR and uses Boot.ini, add /NoSerialMice:COMx to the operating-system boot entry, replacing COMx with the UPS’s actual port—for example, /NoSerialMice:COM1. Back up Boot.ini first, then restart. This stops Windows from probing that port for a serial mouse; it does not disable the port or install UPS support.
Compatibility: This is a legacy Windows NT/2000-era fix, not a normal Windows 10 or 11 setting. Microsoft says the built-in Windows UPS service does not support COM-port UPS units in Windows Vista and later; use a supported USB or network connection where possible (Microsoft’s UPS minidriver documentation).
Why Windows mistakes a UPS for a mouse
Some older UPS units communicate over RS-232 serial. During startup, legacy Windows versions can probe COM ports for serial pointing devices. A device that continuously sends data may be misidentified as a serial mouse, and mouse support can interfere with UPS software trying to use that port. Microsoft documented this behavior for Windows 2000 and earlier NT-family systems (Microsoft Knowledge Base archive).
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The boot option controls serial-mouse detection only. It does not turn off the physical COM port, change the UPS protocol, provide a driver, or guarantee that the UPS software supports your model.
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Check that this procedure fits your computer
- Windows NT, Windows 2000, or another legacy installation using
Boot.ini: The switch may apply. Confirm that the computer actually uses the olderNTLDRboot configuration. - Windows Vista or later: Do not expect this legacy switch or the built-in Windows UPS service to enable a serial UPS. Use a supported USB connection, compatible vendor software, or network management instead.
- UPS connected by USB: Serial-mouse detection is generally not the issue. Check the USB connection, driver, and management software.
Find the correct COM port
Use the port selected in the UPS software and cross-check it against the computer’s hardware listing, Device Manager where available, physical port label, expansion-card configuration, or UPS documentation. A native connector labeled COM1 is not necessarily the port used by a USB-to-serial adapter: Windows may assign an adapter COM3, COM4, or another number.
The number in the switch must match the port Windows probes. If the UPS is on COM2, use /NoSerialMice:COM2, not COM1. A wrong number can leave the problem in place or suppress mouse detection on an unrelated port.
Back up and edit Boot.ini
- Back up
C:Boot.inito a safe location before changing it. A syntax error can affect boot selection. - If needed, make hidden and system files visible in Explorer. If Windows prevents editing, temporarily remove the file’s read-only, hidden, and system attributes; restore the original attributes afterward.
- Open
Boot.iniin a plain-text editor and find the operating-system entry you actually use. - Add a space and the targeted switch to the end of that entry’s existing options. Do not replace the line or alter its
multi(),disk(),rdisk(), orpartition()values. - Save the file, restore its original attributes if you changed them, and restart.
- After startup, test whether the UPS software can open the port and report UPS status.
For example, a Windows 2000 entry might look like this; the path, label, and existing options vary by installation:
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multi(0)disk(0)rdisk(0)partition(1)WINNT="Microsoft Windows 2000 Professional" /fastdetect /NoSerialMice:COM1Microsoft’s archived instructions describe adding the option to the relevant boot entry, restoring file attributes, and restarting (archived Microsoft KB article).
Choose a targeted or global setting
For a UPS, the targeted form is usually the safer choice:
/NoSerialMice:COM1It stops serial-mouse detection on COM1 while leaving detection on other COM ports available. The documented forms include:
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/NoSerialMice:COM1— exclude COM1./NoSerialMice:COM1,2,3— exclude COM1, COM2, and COM3./NoSerialMice— disable serial-mouse detection on all COM ports.
Use the global form only when no serial mouse or other pointing device depends on Windows detecting one. It can break a real serial mouse on another port.
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Multi-boot systems and existing options
Each operating-system line in Boot.ini is a separate boot entry. Add the switch to the entry used for UPS communication; changing an unused entry has no effect on the current installation. If multiple legacy Windows installations use the UPS, update each applicable entry.
Older APC PowerChute installations could add the switch automatically. Inspect the existing entry before editing: if the option is already present with the wrong port, correct it rather than adding a second, potentially conflicting version.
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If the UPS still is not detected
- Verify the cable first. A connector that fits is not proof that the cable has the right wiring or signaling. APC warns that Smart-UPS serial cables can be proprietary and that an incorrect cable can cause serious problems, including unexpected UPS output behavior. Match the cable to the exact UPS model and software (APC cable guidance).
- Recheck the COM number. Confirm the port in the UPS software and Windows. USB-to-serial adapters can receive a different or changing COM assignment.
- Check the adapter driver. Some adapters require a vendor-specific driver. APC documents one setup using a 940-0272A cable in which installing the Prolific PL2303 driver and restarting resolves discovery problems; this applies to that documented configuration, not every adapter (APC adapter guidance).
- Release the port from competing software. Close terminal or diagnostic tools and stop competing UPS utilities, modem services, or other software that may already have opened the COM port. A mouse-detection switch cannot make a port available if another program owns it.
- Check model and signaling compatibility. Legacy UPS software may expect a particular simple or smart signaling mode. On older Windows versions, smart-signaling units can require a model-specific UPS minidriver. Confirm compatibility among the UPS, cable, software, adapter, and operating system.
- Confirm the Windows generation. The built-in Windows UPS service’s serial support is a legacy feature. Windows Vista and later do not support COM-connected UPS units through that service; a third-party utility does not automatically make every serial UPS or adapter compatible.
APC’s current software guide describes model-dependent USB, serial, and network management options, including PowerChute Serial Shutdown and PowerChute Network Shutdown. Check the supported-products information for your exact UPS before choosing software (APC UPS management software guide).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Advanced Windows 2000 case
If a device is still being identified as a mouse despite the boot option, Microsoft documented a separate Windows 2000 workaround involving the SkipEnumerations registry value under the affected device’s hardware-enumeration key (Microsoft Knowledge Base archive). Treat this as a version-specific, advanced measure—not the first-line UPS fix. The registry path depends on the enumerator, device ID, and instance ID, so a generic copy-and-paste registry command is unsafe. Back up the registry and identify the exact device key before considering changes.
Modern options for a serial-only UPS
- Use native USB if your UPS model supports it, together with management software that lists the model as compatible.
- Use network management for supported UPS models with a network management card, especially when several servers need coordinated shutdown.
- Use a documented adapter and cable if serial is unavoidable. Verify support for the UPS model, operating system, software, and signaling mode rather than relying on connector shape alone.
- Keep a dedicated legacy host when an irreplaceable UPS supports only older serial software. This can be more practical than trying to add unsupported serial UPS support to a current Windows PC.
If editing Boot.ini prevents startup
Boot into a recovery or installation environment appropriate for that Windows version and restore the backup. If the rest of the entry is valid, remove only the newly added /NoSerialMice... option. Restart and test the UPS cable, COM assignment, drivers, and software separately.
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Frequently Asked Questions
Does this work on Windows 10 or Windows 11?
It is a legacy Boot.ini option for systems using the older NT loader, not a normal Windows 10 or 11 setting. For a modern PC, use a supported USB or network UPS connection and compatible vendor software.
Will this disable my real serial mouse?
The targeted form disables serial-mouse detection only on the named port. The bare /NoSerialMice form disables it on all COM ports and may stop Windows detecting a serial mouse elsewhere.
Does /NoSerialMice:COM1 disable COM1?
No. It prevents serial-mouse detection on COM1; it does not turn off the port. UPS software can still use it if the cable, driver, software, and port assignment are compatible.
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No. Edit the entry used by the operating system that communicates with the UPS. Update other entries only if those installations also need the setting.
Can I use any DB9 cable with an APC UPS?
No. APC warns that Smart-UPS serial cables may use proprietary wiring and signaling. Use a cable specified for the exact UPS model and intended software.
What if a USB-to-serial adapter appears as COM4?
Configure the UPS software for the adapter’s actual assigned port and confirm which port Windows probes. Do not assume it is COM1 because the computer has a physical COM1 connector.
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