When the X server is automatically configured, users with serial mice must perform additional manual configuration, specifically: 1. [only for releases prior to 6.06/ Dapper] Install the 'joystick' package (don't ask) 2. Run 'inputattach -help' and find the appropriate protocol option to match your mouse 3. Run 'inputattach /dev/tty S0'. This command must be run at system startup in order for this configuration to be permanent; in Dapper, it may be added to /etc/rc.local.
It turns out that mouse detection in Windows is normally handled by the serenum.sys filter driver. This driver implements support for legacy serial mice along with serial plug-and-play. Microsoft has even provided the sourcecode as a WDK sample. During detection the ports switches to 1200-7- N-1 mode while asserting DTR+ RTS to which a response is expected within 200 ms, with a couple of retries in case of failure. Unfortunately for a legacy mouse a single M or B character suffices as identification. In our case the protocol was reworked to avoid these characters and now appears not to be misidentified anymore. However we were using a virtual USB serial port and for a traditional serial port this approach may be somewhat difficult as anything sent at a different baud rate is liable to look like line noise. In this case I suppose the easiest workaround is probably, as has already been suggested, to avoid making any unsolicited transmissions. Alternatively with the serial control signals actually hooked up, or intercepted by a USB CDC device, processing the DTR or RTS signals and holding off on output. Actually implementing the plug-and-play protocol would be an even niftier option. Supposedly there are cheap RS232 cables around without a full complement of control signals though so this approach might still fail.
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