Howto add serial port to WL-500gx/gp

[ryzhov_al]

Still one question, is it so that the serial port cannot be used if terminal output is on (left as "console=ttyS0,115200")?

Sorry, can't test for now.

And if I set it to console=null, would that make it impossible to flash the Asus from serial?

No! Bootloader still works. But if you don't believe me, you can always clean NVRAM settings by Reset button.

Or does it only switch off terminal output after the bootloader, or what?

Yes. Bootloader is still using those port. So your Arduino device must ignore some garbage and wait till router finish booting. console=null is a kernel switch, so port will be silent only when CFE starts kernel.