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2.1
Termination Resistors
Termination resistors are used to match impedance of the network to the impedance of the
transmission line being used. When impedance is mismatched, the transmitted signal is not
completely absorbed by the receiver and a portion of signal is reflected back into the transmi
ssion line.
The decision whether or not to use termination resistors should be based on the cable lengt
h and data rate used by the communication system.
For example, if you use 9,600 baud rate and 1,200m length of cable, the propagation veloci
ty of cable is 0.66 x speed of light (This value is specified by the cable manufacturer), if we a
ssume the reflections will damp out in three round trip up and down the cable length, the tr
ansmitted signal will stabilize 18.6us after the leading edge of a bit. Since the data bit is cap
tured in the middle of the bit which is approximately 52us after the leading edge of a bit. Th
e reflection stabilizing time 18.6us is much before the center of the bit therefore the termin
ation resistors are not required.
However, if you install the cable to maximum length, the impedance of cable and network is
mismatched and the transmitted signal is overlapped by the reflected signal. In this case, it is
recommended to add termination resistors to the end of the receiver lines. A 120Ω resistor c
an be used for termination resistor in parallel between the receiver lines "A" and "B" for 2 wir
es RS485 system or "RX+" and "RX-" for 4 wires RS422 system. A termination resistor of less
than 90Ω should not be used and no more than 2 terminations should be used in one netw
orking system.
2.2
How to Connect Termination Resistors
Termination Resistors for 2 Wire RS485 Communication System
p.16
Installation Checkpoints & Tips
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