How can I switch temporally Linux ASCII console to RS232 port build-in computer. I mean about Linux system working as server, without running any graphical tools on it (X11, bootsplash etc). But I want to do it temporally just "on demand". In ordinary usage, the console should be normal VGA monitor. Is it possible?
Temporarily? Too much of a hassle.
It's all well documented in the kernel documentation, and in modern distributions it should be sufficient to edit
/etc/inittab in order to have a terminal listening on your serial port - which is what I assume you want to do.
If you just want the console messages displayed, edit your boot options accordingy.