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Termination resistors are used to eliminate signal reflections in communication cables during transmission. There are two factors that cause signal reflections: impedance discontinuity and impedance mismatch. Due to the impedance discontinuity, the signal encounters a cable impedance that is very low or even non-existent at the end of the transmission line; this causes the signal to reflect at that point. The principle of this signal reflection is similar to the reflection that occurs when light passes from one medium into another. To eliminate this reflection, it is necessary to connect a termination resistor at the end of the cable with a value equal to the cable’s characteristic impedance, thereby ensuring continuity of the cable’s impedance. Since the transmission of signals over a cable is bidirectional, a terminal resistor of the same value can be connected at the other end of the communication cable. Another cause of signal reflection is the impedance mismatch between the data transceiver and the transmission cable. The reflection caused by this reason is mainly manifested as chaos in the network data when the communication lines are in the idle mode. To reduce the impact of reflected signals on communication lines, noise suppression and the use of bias resistors are commonly employed. In practical applications, for relatively weak reflection signals, the method of adding a bias resistor is often used for simplicity and convenience. Additional notes: 1. RS-485 requires 2 termination resistors, which should be connected to both ends of the transmission bus; their value must be equal to the characteristic impedance of the transmission cable. Termination resistors are not required for short-distance transmission; in general, they are not needed for distances under 300 meters. 2. To suppress interference, a 120-ohm resistor is often connected to the RS485 bus after the last device (as mentioned above). 3. The common-mode output voltages of RS-485 and RS-422 are different. The common-mode output voltage for RS-485 is between -7V and +12V, while that for RS-422 is between -7V and +7V. The minimum input impedance of an RS-485 receiver is 12KΩ ； RS-422 is 4kΩ ； RS-485 meets all the specifications of RS-422, so RS-485 drivers can be used in RS-422 networks. This post was last edited by Mobei Yihai on 2009-4-3 10:27.]