This is off-topic.... I learned the difference between RS232C and RS485 communications.

RS-232C

• Single end signal (week for noise)
• If you use large amplitude voltage(±5V~15V), you can avoid the noises.
• It is bad for long way communication, the max length is 15m.

RS-422

• Strong for noise because it has differential signal system.
• The amplitude voltage is lower than RS-232(±2～5V)
• It can communicate faster than RS-232(The maximum transmission speed is about 10Mbps)

RS-485

• Strong for noise because it has differential signal system.
• It is an improved version of RS-422 with enhanced functions.
• RS-485 addresses the multi-drop feature limitation in RS-422
• Up to 32 devices can be used on the same data line

I still don't know what the single end signal, differential signal and multi-drop.