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iCON100/iCON100-SR QUICK INSTALLATION GUIDE
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4.2 Check Points during Installation
4.2.1 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 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.
4.2.2 How to Connect Termination Resistors
4.2.3 Grounding System for Communication Cable
We recommend to using proper grounding system on the communication cable. The best method for
grounding system is to put the shield wire of the communication cable to the 1
st
class earth grounding;
however it is not so easy to bring the earth ground to the communication cable and also the installation cost is
raised.
There will be three grounding point where you can find during installation;
1)
Earth Ground
2)
Chassis Ground
Figure 1 Termination resistors for 4 Wire RS422 Communication System
Figure 2 Termination Resistors for 4 Wire RS485 Communication System
