IDTECK Star Finger007, Manual

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p.22 Check Point during Installation
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
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 l
ess than 90Ω should not be used and no more than 2 terminations should be used in one ne
tworking system.
2.2 How to Connect Termination Resistors
Termination Resistors for 2 Wire RS485 Communication System
Termination Resistors for 4 Wire RS422 Communication System
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