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RS-485: Transmitting Full-Duplex Data over Single Twisted-Pair Cable Application Note
Figure 5
shows a best-fit line drawn through the driver V-I characteristic curve. The slope of this line represents
R
. In this case, the output impedance of the RAA788153 transceiver is derived with R
O
V
S
D
R
D1
Z
IN
Figure 4. Bus Node Impedance Matching
When the value of R
O
2R
+
S
(EQ. 3)
--------------------------------------------
R
=
T
2R
+
S
1.3
4-to-2 Wire Conversion
The 4-to-2 wire conversion not only passes transmit signals from the driver to the bus and receive signals from the
bus to the receiver, but it must also cancel the portion of the transmit signal that is unavoidably looped back to the
receiver
(Figure
6).
Because of the design structure of the 4-to-2 wire conversion, a part of the transmit signal is looped back into the
receiver path, where it is superimposed onto the receive signal. If not removed, the combined signal can cause
false triggering of the receiver input thresholds, therefore, leading to data errors.
The canceling of the loopback signal is achieved by feeding an inverted portion of the transmit signal to the
receiver inputs, where it adds to the loopback signal. To eliminate the loopback signal, the canceling signal must
be of the same amplitude but inverted polarity as the loopback signal.
Inverting the canceling signal is accomplished by connecting the non-inverting receiver input (A) to the inverting
driver output (Z), and the inverting receiver input (B) to the non-inverting driver output (Y). The amplitude of the
canceling signal is set by the resistor ratio of the R D -R B voltage divider, which, in turn, depends on the ratio of bus
voltage to driver output voltages.
R15AN0009EU0100 Rev.1.00
Mar 4, 2022
R
S
Y
Z
R
O
T
Z
R
S
= R
||(2R
+ R
)
= Z
T
S
O
0
is known, calculate the value of the termination resistor by solving
Z
R
1
O
0
---------------------------------------------------------- -
=
R
–
Z
1 Z
–
1
O
0
0
A
V
R
AB
B
R
R
IN
IN
Z
V
D
OD
Y
Figure 6. Deriving Driver Output Impedance
0
Figure 5. Deriving Driver Output Impedance
2R
+
R
S
O
R
B
Receive
Loopback
Σ
R
B
R
R
D
D
R
S
Transmit
R
S
≈ 25Ω.
O
140
120
RAA788153
100
80
R
~ 25Ω
O
60
40
20
0
0
1
2
3
Driver Output Voltage (V)
Equation 3
P
R
V
T
Bus
N
4
5
for R
:
T
Page 3
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