IDTECK Star FINGER006, User Manual

Page: 15 / 51

15
7.2 Check Point during Installation
7.2.1 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 120
Ω
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 90
Ω
should not be used and no more than 2 terminations should be
used in one networking system.
7.2.2 How to Connect Termination Resistors
Figure: Termination Resistors for 2-Wire RS485 Communication System
Figure: Termination Resistors for 4-Wire RS422 Communication System