User Manual

RS-485 Network
E-
6 ADAM 4000 Series User’s Manual
Figure E-5 Termination resistor locations
Because each input is biased to 2.4 V, t he nom inal common mode
voltage of bal anced RS-485 systems, the 18 k Ω o n t he i nput ca n be
taken as being in series across the input of each individual receiver.
If t hirty of t hese recei vers are put cl osely toget her at the e nd of t he
transmission line, they will te nd to react a s thirty 36kΩ resistors in
parallel with t he term ination resisto r. Th e overall effecti ve resistan ce
will need to be close to the characteristics of the line.
The effective parallel receiver resistance RP will therefore be equal to:
R
P
= 36 x 10
3
/30 = 1200 W
While the termination receptor R
T
will equal:
R
T
= R
O
/ [1 - R
O
/R
P
]
Thus for a line with a ch aracteristic i mpedance of 100 Ω resistor, t he
termination resistor R
T
should be:
R
T
= 100/[1 - 100/1200] = 110 Ω
Since t his value lies within 10% of th e line ch aracteristic im pedance.
Thus as alrea dy stated a bove the line te rmination resi stor R
T
will
normally equal the characteristic impedance Z
O
.
The star con nection causes a multitude o f th ese d iscontinuities sin ce
there are several transmission lines and is therefore not recommend.
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.