"Are there monitors available that works on serial port?"

Yes - they're called "Terminals" or, more precisely, Video Display Terminals - VDTs (aka VDUs).

A VDT also has a keyboard - but you don't have to use it if you don't want to!

A Terminal Emulator is a software application which runs on, say, a PC and performs the functions of a terminal: it receives data from the PC's serial port & displays it on the PC's screen (it will also receive characters typed at the keyboard & send them out to the serial port).

The Win9x "Hyperterminal" is a Terminal Emulator - though not universally well regarded! ;-)