Datasheet
MAX1480A/B/C, MAX1490A/B
Complete, Isolated RS-485/RS-422
Data Interface
______________________________________________________________________________________ 19
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).
2
8
3kΩ
3kΩ
3kΩ
5712
5712
11
23
13 1
3kΩ
2
43 13
3kΩ
74HC04
200Ω
9
25
25
23
26
A
B
A
B
19
9
200Ω
200Ω
DRIVER
ENABLE
B > A
200Ω
74HC123
CLR
4
AQ
311
1
DRIVER
ENABLE
A > B
11
22
17
200Ω
15
21
24 24
10
14
NETWORK SEGMENT A
NETWORK SEGMENT B
2 8 10 14
CLR
74HC123
12AQ9
3kΩ
26
19
22
17
200Ω
15
21
1000pF51kΩ
+5V
+5V +5V
+5V
51kΩ
16 15
14
13
2B Q
1000pF
7
6
5
10
BQ
MAX1480C MAX1480C
Figure 15. Doubly Isolated RS-485 Repeater