The higher the data rate and the longer the cable, the more likely it will be that you have to add termination resistors. Lower data rates have more time for the ringing to stabilize so the lower the data rate the less needed this resistor is. Since data rates on a PC serial port can vary from 300 baud (26mS for 8-bits) up to over 100k baud (0.08mS for 8-bits), the timer will have to either be limited in the baud rates that it can work with, or keep control of the RS-485 network for much longer than it needs. From a hardware point of view, full-duplex RS-485 has some major advantages over RS-232 -- it can communicate over much longer distances at higher speeds. A long 3-conductor cable intended for RS-232 can often be switched to half-duplex RS-485, allowing communication at higher speeds and RS485 standard at higher external noise levels than the same cable used with RS-232 signaling.