MAX1480A_05 Datasheet, PDF (19/21 Pages) Maxim Integrated Products – Complete, Isolated RS-485/RS-422 Data Interface

Complete, Isolated RS-485/RS-422
Data Interface
Connections and components from one side should
not be located 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 MAX1480A/MAX1480B/MAX1480C/MAX1490A/
MAX1490B are specified and characterized using the
resistor values shown in Table 1 of Figure 1 and Table
2 of Figure 2. Altering the recommended values can
degrade performance.
The DI and DE (MAX1480A/B/C 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 resis-
tor values shown in Tables 1 and 2 are recommended
when the 74HC86 gate or equivalent is used. These
values may need to be adjusted if a driving gate with
dissimilar series resistance is used.
All pull-up 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 will cause the iso-
lated common voltage to go to the mean voltage of the
receiver inputs. If using shielded cable, connect the
isolated common to the shield through a 100Ω resistor.
In the case of the MAX1490, each shield should have
its own 100Ω resistor (Figures 1, 2, 13, and 14).
NETWORK SEGMENT A
A 23
B ²⁵
26
3kΩ
200Ω
3kΩ
19
22
17
21
24
15
+5V
2 8 10 14
3kΩ
13
1
2
+5V
200Ω
2 8 10 14
9
MAX1480C
200Ω
9
74HC04
4
3
200Ω
11
DRIVER
ENABLE
B>A
3kΩ
13
DRIVER
ENABLE
A>B
200Ω
11
MAX1480C
5 7 12
+5V _{51kΩ 1000pF}
16
2B
15 14
Q 13
74HC123
1A
Q4
CLR
3
5 7 12
+5V _{51kΩ 1000pF}
10 B
76
Q5
74HC123
9A
Q 12
CLR
11
NETWORK SEGMENT B
23 A
25
B
26
3kΩ
19
22
3kΩ
17
21
24
200Ω
15
Figure 15. Doubly Isolated RS-485 Repeater
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