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HELP2
If the noise problem is serious, protect the controller by using
termination resistors or shield wires.
1) Termination Resistor
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 transmission line.
The decision whether or not to use termination resistors should be based on the cable length 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 velocity of cable is
0.66 x speed of light (This value is specified by the cable manufacturer), if we assume the reflections will
damp out in three round trip up and down the cable length, the transmitted signal will stabilize 18.6us after
the leading edge of a bit. Since the data bit is captured in the middle of the bit which is approximately 52us
after the leading edge of a bit. The reflection stabilizing time 18.6us is much before the center of the bit
therefore the termination 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 can be used for termination resistor in
parallel between the receiver lines "A" and "B" for 2 wires 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 networking system. Refer to the following picture for information about how to install
termination resistors.
Installation of termination resistors.
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RS422 Communication
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RS485 Communication
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