Look into rs485; it is differential pair signaling, and will give good noise immunity over that distance. It is also easy to do - use a max485 instead of a max232 driver.
Also if you need to go over the max length you can make a box with rx-tx pairs to extend, you just need a power source at that point. A place I worked for did this for special orders and we would have to run the system with the full cable length and monitor data errors and data retrans.
I searched but there is no data sheet for RS422.Did you find any data sheet?
The other thing is I'm confuse with "what is the meaning of balance line & unbalance line" in communication.In RS232 I don't know what is that balance or unbalance.