SN65HVD72
SN65HVD75
SN65HVD78
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SLLSE11B –MARCH 2012–REVISED JUNE 2012
Isolated Bus Node Design
Many RS-485 networks use isolated bus nodes to prevent the creation of unintended ground loops and their
disruptive impact on signal integrity. An isolated bus node typically includes a micro controller that connects to
the bus transceiver via a multi-channel, digital isolator (Figure 23).
0.1μF
2
MBR0520L
1:1.33
3.3V
ISO
3
1
4
1
2
Vcc
D2
IN
OUT
TLV70733
EN GND
SN6501
10μF 0.1μF
10μF
3
D1
GND
4,5
10μF
MBR0520L
L1
N
ISO-BARRIER
3.3V
0.1μF
0.1μF
PSU
PE
0.1μF
0.1μF
1
16
4.7k
4.7k
Vcc1
Vcc2
PE
2
7
6
3
4
5
10
11
14
13
12
8
EN1 ISO7241 EN2
DVcc
16
11
12
15
1
2
3
4
Vcc
OUTD
INA
IND
OUTA
OUTB
UCA0RXD
P3.0
R
R1
R2
5
6
7
6
XOUT
XIN
B
A
RE
MSP430
F2132
SN65
HVD72
INB
P3.1
DE
D
INC
OUTC
GND2
9,15
UCA0TXD
DVss
4
GND2
5
GND1
2,8
TVS
R
C
HV
HV
Short thick Earth wire or Chassis
PE
island
Protective Earth Ground,
Equipment Safety Ground
R1,R2, TVS: see Table 1
= 1MΩ, 2kV high-voltageresistor, TT electronics, HVC 2010 1M0 G T3
= 4.7nF, 2kV high-voltagecapacitor, NOVACAP, 1812 B 472 K 202 N T
R
HV
C
HV
Floating RS-485 Common
Figure 23. Isolated Bus Node With Transient Protection
Power isolation is accomplished using the push-pull transformer driver SN6501 and a low-cost LDO, TLV70733.
Signal isolation uses the quadruple digital isolator ISO7241. Notice that both enable inputs, EN1 and EN2, are
pulled-up via 4.7k resistors to limit their input currents during transient events.
While the transient protection is similar to the one in Figure 22(left circuit), an additional high-voltage capacitor is
used to divert transient energy from the floating RS-485 common further towards Protective Earth (PE) ground.
This is necessary as noise transients on the bus are usually referred to Earth potential.
RVH refers to a high-voltage resistor, and in some applications even a varistor. This resistance is applied to
prevent charging of the floating ground to dangerous potentials during normal operation.
Occasionally varistors are used instead of resistors in order to rapidly discharge CHV, if it is expected that fast
transients might charge CHV to high-potentials.
Note that the PE island represents a copper island on the PCB for the provision of a short, thick Earth wire
connecting this island to PE ground at the entrance of the power supply unit (PSU).
In equipment designs using a chassis, the PE connection is usually provided through the chassis itself. Typically
the PE conductor is tied to the chassis at one end while the high-voltage components, CHV and RHV, are
connecting to the chassis at the other end.
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