Datasheet
Vcc1 Vcc2
GND1 GND2
OUTA
16
14
13
2,8 9,15
INA
OUTD
1
3
4
5
6
ISO7241
0.1μF 0.1μF
EN1 EN2
7 10
INB
12
11
OUTB
OUTC
IND
INC
4.7k
4.7k
10μF 0.1μF
MBR0520L
MBR0520L
1:1.33
0.1μF
3
1
D2
SN6501
D1
Vcc
4,5
2
GND
3.3V
IN
EN GND
OUT
4 1
23
TLV70733
10μF
3.3V
ISO
10μF
ISO-BARRIER
8
7
6
5
UCA0RXD
P3.0
P3.1
UCA0TXD
16
11
12
15
4
XOUT
XIN
5
6
2
MSP430
F2132
1
2
3
4
0.1μF
TVS
R1
0.1μF
DVss
DVcc
R2
Vcc
GND2
B
A
R
RE
DE
D
SN65
HVD72
R
HV
PSU
L1
N
PE
PE
PE
island
Protective Earth Ground,
Equipment Safety Ground
Floating RS-485 Common
R
HV
R1,R2, TVS: see Table 1
Short thick Earth wire or Chassis
C
HV
C
HV
= 1MΩ, 2kV high-voltage resistor, TT electronics, HVC 2010 1M0 G T3
= 4.7nF, 2kV high-voltage capacitor, NOVACAP,1812 B 472 K 202 N T
SN65HVD72
SN65HVD75
SN65HVD78
www.ti.com
SLLSE11C –MARCH 2012–REVISED SEPTEMBER 2013
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).
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.
R
VH
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 C
HV
, if it is expected that fast
transients might charge C
HV
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, C
HV
and R
HV
, are
connecting to the chassis at the other end.
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