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RS-485: Transmitting Full-Duplex Data over Single Twisted-Pair Cable Application Note
Then, applying
Equation
R
=
R
k
B
D
1.7
Deriving the Driver Output Voltage, V
Knowing the magnitude of V
calculate the load resistance of the driver in
R
2R
R
=
+
(EQ. 12)
L
S
Then, drawing the R
line into the driver V-I characteristic, V
L
(Figure
13).
D1
R
L
Figure 12. Driver Load Resistance
2.
Calculation Examples
Table 1
lists the sequence of steps for calculating the gain and resistor values for 100Ω and 120Ω Cables.
Parameter
Characteristic Cable
Impedance
Driver Output
Impedance
Receiver Input
Impedance
Current Limiting
Resistor
Termination Resistor
Forward Gain
Driver Loopback
Resistor
Bus Resistor
R15AN0009EU0100 Rev.1.00
Mar 4, 2022
10, calculate the resistor value for R
helps to estimate the receiver input voltage during circuit simulation. To find V
OD
Figure 12
1
---------------------------------- -
Z
2R
=
+
T
0
S
1 R
+
T
R
S
R
R
Z
L
T
0
R
S
= 2R
+ R
|| Z
S
T
0
Table 1. Sequence of Calculation Steps
Symbol
Z
RS-485 or CAT-5
0
Established from the driver V-I characteristic in the
R
O
data sheet
Taken from the Electrical Specifications in the data
R
IN
sheet
Z
0
R
------
R
=
S
S
2
---------------------------------------------------------- -
R
=
R
T
T
1 Z
–
0
------------------------------------------------------------- -
k
=
k
1
+
2R
S
R
R
=
20
R
D
D
R
=
R
k
R
B
D
B
:
B
OD
with:
1 Z
0
is found at the crossing point of both curves
OD
Figure 13. Deriving V
Equation / Comment
Calculated
E-96 series
Calculated
1
1
2R
+
R
S
O
E-96 series
1
1 Z
+
1 R
0
T
Calculated
2 R
+
2R
T
T
S
E-96 series
Calculated
E-96 series
140
120
RAA788153
100
80
60
40
R
20
L
0
0
1
2
3
Driver Output Voltage (V)
for a given R
OD
Value
120
25
96k
60
59
746
750
0.4671
10.475k
10.5k
4.905k
4.87k
,
OD
V
OD
4
5
L
Unit
100
Ω
25
Ω
100
Ω
50
Ω
49.9
Ω
503
Ω
499
Ω
0.455
7.045k
Ω
6.98k
Ω
3.176k
Ω
3.16k
Ω
Page 7
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