[syslinux] disabling serial in pxelinux

[DI. Thomas Schallar]

Hello from Vienna, Austria!

I'm happily using pxelinux since years. However, I encountered problems 
on several machines, because they don't boot up properly with PXE. The 
problem is, that there are recently some devices connected to the serial 
ports of those machines. pxeboot echoes and listens to the serial 
console and the output of the menu results in (silly) input back from 
the serial port. That is considered as user input and stops the timeout 
counter: the machine won't continue booting. :-(

Since I can't remove the serial devices I have only two solutions:

   1. disabling the serial port in pxeboot (or at least ignoring input
      from it)
   2. disabling PXE boot (won't like that - it's a perfect remote
      control in case of OS problems)

To disable the serial console I tried to play with the "serial" 
configuration option (http://syslinux.zytor.com/faq.php#CMD_SERIAL), but 
with no result yet. I tried

    * serial 2 9600 - to use a nonexistent serial port (ports 0 and 1 do
      exist and both have devices connected to them)
    * serial 0 9600 0x400 - to ignore input unless RI is asserted -
      which won't happen, so input should be ignored permanently

Neither solution worked. Is there some

    serial off
    console 2
    serialconsole 0

or other means to completely turn off the communication with any serial 
port? Any help is appreciated!

greetings,
Thomas