MAX1480 Maxim, MAX1480 Datasheet - Page 18

• Manufacturer Part Number: MAX1480
• Description: Complete / Isolated RS-485/RS-422 Data Interface
• Manufacturer: Maxim

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.
Complete, Isolated RS-485/RS-422
Data Interface
Figure 15. Doubly Isolated RS-485 Repeater
18
NETWORK SEGMENT A
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.
______________________________________________________________________________________
3k
200
Pull-Up and LED Drive Resistors
A
B
3k
23
25
26
15
19
22
17
21
24
MAX1480C
2 8
+5V
5 7 12
10 14
11
13
9
2 B
1
+5V
A
200
200
16
74HC123
51k
CLR
3k
3
1000pF
DRIVER
ENABLE
B > A
15
Q
Q
14
13
4
4
1
74HC04
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.
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, tie 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).
3
2
10 B
+5V
9
A
DRIVER
ENABLE
A > B
51k
74HC123
CLR
3k
11
200
1000pF
7
200
Q
Q
6
5
12
9
13
11
Isolated Common Connection
2 8 10 14
5 7 12
MAX1480C
+5V
19
22
17
21
15
23
25
26
24
3k
200
A
B
NETWORK SEGMENT B
3k