RS-485 Network
E-6
MIR 6000
Series User’s Manual
Figure E-5
Termination resistor locations
Because each input is biased to 2.4 V, the nominal common mode
voltage of balanced RS-485 systems, the 18 k
Ω
on the input can be
taken as being in series across the input of each individual receiver.
If thirty of these receivers are put closely together at the end of the
transmission line, they will tend to react as thirty 36k
Ω
resistors in
parallel with the termination resistor. The overall effective resistance
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
Ohm
While the termination
receptor
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.
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.
