I want to build a network, in this network there is only one PC and 50 terminals, 50 students can use this system to study UNIX/Linux.
In the old days, computer was very expensive, many scientists shared a computer with terminals, that means a terminal has no cpu, memory and hardisk.
In these days, how to use a PC(UNIX/Linux) to support such text / graphics terminals?
Thanks a lot!
For plain text, just tell the system to run a login program on /dev/ttyS* by adding it to /etc/inittab. A description can be found here.
For a graphical connection, you'd need to set up a BOOTP server for basic configuration, a tftp server for the most basic files, and configure the XDMCP server to advertise its service on the network.
Thank you!
Which terminals you actually intend to use/buy? Text-terminal or Thin Client?
However I'm sure you will find following very interesting to read: Text-Terminal-HOWTO
Also interesting: Serial HOWTO: Multiport Serial Boards/Cards/Adapters
If you're gonna set it up with text-terminals, prepare for using a couple of these :D:
Otherwise if you use Thin Clients, you'll need something like this:
Good luck! 
Yeah, it's somewhat paradoxical. These days network hardware is cheaper and more easily available than less-capable serial hardware. 
Thank you, every one.
My origin idea is too naive.
I want each client can run light graphic program, like cash register, the speed and length is the main problem, only ethernet(cheep) can do that. Serial connection is slow and short, can't do much help.
At last I have to choise the thin client, made by embedded system device, connected by ethernet.