Complete Interface Circuit And Waveforms - Renesas RS-485 Quick Start Manual

RS-485: Transmitting Full-Duplex Data over Single Twisted-Pair Cable Application Note
Parameter
Resistor Ratio
Gain Error
Driver Load Resistance
Driver-2 Output Voltage
Receiver-1 Input
Voltage
3.

Complete Interface Circuit and Waveforms

Figure 14 shows the complete interface circuit with a table listing the selected resistor values for bus nodes with
120Ω and 100Ω input impedance. The 1Mbps full-duplex transceivers, RAA788153, are permanently enabled by
connecting the receiver enable pin, RE, to ground and the driver enable pin, DE, to V CC .
5V
C
B
V
CC
A
RO
RO1 R
B
RE
R
XCVR
DE
5V
Z
DI
DI1 D
Y
GND
XCVR = RAA788153
Figure 15 and Figure 16
data input pin of Node 1, DI1, results in a slightly delayed output of the same data rate at the receiver output of
Node 2, RO2. In the opposite direction, applying a 400kHz (800kbps) square wave to the data input pin of Node 2,
DI2, results in a slightly delayed output of the same data rate at the receiver output of Node 1, RO1.
The noisy signals in Figure
between the nodes input impedance of Z
noise free signals in Figure 16
an input impedance of Z
R15AN0009EU0100 Rev.1.00
Mar 4, 2022
Table 1. Sequence of Calculation Steps (Cont.)
Symbol
R /R
B D

R
R
B D
ε
 ------------------- -
=
k
R
R = 2R +
L
L S
Established from the driver V-I characteristic in the
V
OD2
data sheet
V
V
AB1
AB1
Node 1
R
B
R
B
R R
D D T
R
S
R
S
Figure 14. Complete Interface Circuit
depict the input and output waveforms. Applying a 100kHz (200kbps) square wave to the
15, which also show overshoot and cross-coupling, are because of the mismatch
= 120Ω and the characteristic cable impedance of Z
IN
reflect the effects of impedance matching, when the bus node design is changed to
= 100Ω, therefore, matching Z
IN
Equation / Comment

R Z
T 0
V
OD2
------------------------------------------------------------- -
=
   
1 + R 1 R + 1 R
D B IN
Z = 120Ω 100Ω
IN
R = 59Ω 49.9Ω
S
R = 750Ω 499Ω
T
R = 10.5kΩ 6.98kΩ
D
R = 4.87kΩ 3.16kΩ
B
P P
N N
100ft CAT-5 Cable
(Z = 100Ω)
0
.
0
Value
0.4638
0.71
221
4.0
1.23
Node 2
R
B
R
B
R R R
T D D
R
S
R
S
XCVR = RAA788153
Unit
0.4577
0.49 %
183 Ω
3.94 V
1.20 V
5V
C
B
V
CC
A
RO
R RO2
B
RE
XCVR
DE
5V
Z
DI
D DI2
Y
GND
= 100Ω. The
0
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