MAX1480E Datasheet, PDF (17/19 Pages) Maxim Integrated Products – 15kV ESD-Protected, Isolated RS-485/RS-422

±15kV ESD-Protected, Isolated RS-485/RS-422
Data Interfaces
V_IN
+5V
C1
C2
22µF
0.1µF
74HC86
OR EQUIVALENT
R1
DI
DI´
DRIVER INPUT
R2
RO
RECEIVER OUTPUT
V_{CC1 1}
V_{CC2 2}
MAX1490EA/
MAX1490EB
24 ^{AC1 (MAKE NO CONNECTION)}
23 AC2 (MAKE NO CONNECTION)
D1 ₃
D2 ₄
GND1 ₅
FS 6
22 ^{ISO VCC1}
21 ^{ISO RO DRV}
20 ^A
19 ^B
120Ω
A
B
SHIELD (OPTIONAL)
Y
120Ω
D
DI
Z
SD ₇
V_{CC3 8}
DI ₉
V_{CC4 10}
18 ^Z
17 Y
16 ^{ISO COM1}
15 ^{ISO DI DRV}
Z
120Ω
Y
R5, 100Ω
SH1
R4
SH2
SHIELD (OPTIONAL)
B
120Ω
R
RO
A
RO ₁₁
GND2 ₁₂
14 ^{ISO V}CC2
13 ^{ISO RO LED R3}
ISOLATION BARRIER
R6, 100Ω
ISOLATED COMMON
NOTE: RESISTORS R5 AND R6 PROTECT
THE MAX1490EA/MAX1490EB FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
TRANSMISSION LINES.
LOGIC GROUND
270pF
4kV
Figure 16. Typical Full-Duplex RS-485/RS-422 Network
MAX1480EC/MAX1490EA/MAX1490EB. Connections
and components from one side should not be locat-
ed near those of the other side.
• A shield trace connected to the ground on each side
of the barrier can help intercept capacitive currents
that might otherwise couple into the signal path. In a
double-sided or multilayer board, these shield traces
should be present on all conductor layers.
• Try to maximize the width of the isolation barrier
wherever possible; a clear space of at least 0.25
inches between ground and isolated common is
suggested.
Pullup and LED Drive Resistors
The MAX1480EA/MAX1480EC/MAX1490EA/MAX1490EB
are specified and characterized using the resistor val-
ues shown in Tables 1 and 2. Altering the recommend-
ed values can degrade performance.
DI and DE are intended to be driven through a series
current-limiting resistor. Directly grounding these
pins destroys the device.
The DI and DE (MAX1480EA/MAX1480EC only) inputs
are the cathodes of LEDs whose anodes are connected
to the supply. These points are best driven by a CMOS-
logic gate with a series resistor to limit the current. The
resistor values shown in Tables 1 and 2 are recommend-
ed when the 74HC86 gate or equivalent is used. These
values may need to be adjusted if a driving gate with dis-
similar series resistance is used.
All pullup resistors are based on optocoupler specifica-
tions in order to optimize the devices’ data-transfer rates.
Isolated Common Connection
The isolated common may be completely floating with
respect to the logic ground and the effective network
ground. The receiver input resistors cause the isolated
common voltage to go to the mean voltage of the receiver
inputs. If using shielded cable, connect the isolated com-
mon to the shield through a 100Ω resistor. In the case of
the MAX1490EA/MAX1490EB, each shield should have its
own 100Ω resistor (Figures 1, 2, 15, and 16).
Double-Isolated RS-485 Repeater
The RS-422/RS-485 standard is specified for cable
lengths up to 4000 feet. When approaching or exceeding
the specified maximum cable length, a ground-potential
difference of several tens of volts can easily develop.
This difference can be either DC, AC, at power-line fre-
quency, or any imaginable noise or impulse waveform. It
is typically very low impedance so that if a connection
between the two grounds is attempted, very large cur-
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