[aprssig] Serial Mouse vs GPS in Windows

Stephen H. Smith wa8lmf2 at aol.com
Fri Nov 2 15:04:03 EDT 2007

The issue of Windows going nuts (with the mouse cursor skittering 
randomly all over the screen) when it is booted with a GPS already 
connected to a serial port just came up on the UI-View mailing list. 

I think the fix for this issue is of sufficient general interest to APRS 
users with Windows that I am reposting my response here.

Bob Burns W9RXR wrote:

>>  The symptom is that the mouse cursor seems to jump 
>> > > all over the screen and I have little control over the cursor. If I 
>> > > boot the computer with the GPS receiver disconnected, and then 
>> > > connect the GPS after the boot sequence completes, the dongle works 
>> > > about 90% of the time. My assumption is that the computer is 
>> > > interpreting the NMEA strings from the GPS as mouse commands. Very frustrating.
>> > >   
> >   

This is caused by Windows "Plug-N-Pray".    Plug-And-Play doesn't just try 
to detect the parallel, serial and USB ports on a computer; it goes beyond 
the port to attempt to identify the printers, modems, mice and other 
devices plugged into these various ports.   

SOMETHING in the NMEA data steam causes Plug-And-Play to falsely 
conclude that a "Microsoft BallPoint Mouse" (an early clamp-on trackball 
for laptops) is connected to the serial port.  Windows then proceeds to 
forward the entire NMEA data stream into Windows' mouse handling 
routines, which make the mouse cursor go insane.  When this happens, 
unplug the offending GPS device and check the Windows Device Manager. 
You will see an entry under "Mice and other pointing devices" for 
"Microsoft Ballpoint Mouse on COMn".   Just delete this device and then 
reconnect the GPS.  

You can PERMANENTLY stop this behavior with a little-known freeware 
utility from Microsoft called "ComDisable".   This patch program, which 
you run just once, stops Windows from probing serial ports at boot to 
identify the attached devices. It DOES NOT prevent the detection of 
serial ports (either real or virtua) themselves.   

A copy of this utility is located on my website at:


Scroll down the page of misc downloadable items and look for   
*ComDisable.msi*   .

This is an installable program in a Windows .MSI installer package that 
requires that the Microsoft Installer application already be installed 
on your system. (This will already be present on most systems. If it 
isn't go to Microsoft's web site and download the freeware installer.)   
The ComDisable program itself is a command line utility that you run 
from a "Command Prompt" a.k.a "DOS Box" in Windows.  It only needs to be 
run once to permanently disable COM port probing. 


Stephen H. Smith    wa8lmf (at) aol.com
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