Appendix E RS-485 Network E-7
Appendix E
NOTICE: The recommended wiring method that causes a minimum
amount of reflection is daisy chaining where all receivers tap from one
transmission line and needs to be terminated only twice.
The effective parallel receiver resistance RP will therefore be equal to:
R
P
= 36 x 10
3
/30 = 1200 W
While the termination receiptor R
T
will equal:
R
T
= R
O
/ [1 - R
O
/R
P
]
Thus for a line with a characteristic impedance of 100 Ω resistor, the
termination resistor R
T
should be:
R
T
= [1 - 100/1200] = 110 Ω
Since this value lies within 10% of the line characteristic impedance. Thus
as already stated above the line termination resistor R
T
will normally equal
the characteristic impedance Z
O
.
The star connection causes a multitude of these discontinuities since there
are several transmission lines and is therefore not recommend.
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