MAX1480A_05 Datasheet, PDF (18/21 Pages) Maxim Integrated Products – Complete, Isolated RS-485/RS-422 Data Interface

Complete, Isolated RS-485/RS-422
Data Interface
Driver Output Protection
There are two mechanisms to prevent excessive output
current and power dissipation caused by faults or by
bus contention. A foldback current limit on the output
stage provides immediate protection against short cir-
cuits over the entire common-mode range (see the
Typical Operating Characteristics). In addition, a ther-
mal shutdown circuit forces the driver outputs into a
high-impedance state if the die temperature rises
excessively.
Propagation Delay Skew
Typical propagation delays are shown in the Typical
Operating Characteristics using the test circuit of
Figure 5. Propagation delay skew is simply the differ-
ence between the low-to-high and high-to-low propaga-
tion delay. Small driver/receiver skew times help
reduce EMI and reflections by maintaining balanced
differential signals.
The receiver skew time, _I t_PLH - t_{PHL I}, is typically under
100ns for the MAX1480A/MAX1490A and under 1µs for
the MAX1480B/MAX1480C/MAX1490B.
The driver skew time is typically 25ns for the
MAX1480A/MAX1490A and 100ns for the MAX1480B/
MAX1480C/MAX1490B.
___________Applications Information
DI and DE are intended to be driven through a
series current-limiting resistor. Directly grounding
these pins destroys the device.
The MAX1480A/MAX1480B/MAX1480C are designed
for bidirectional data communications on multipoint
bus-transmission lines. The MAX1490A/MAX1490B are
designed for full-duplex bidirectional communications
that are primarily point-to-point. Figures 13 and 14
show half-duplex and full-duplex typical network appli-
cation circuits, respectively. To minimize reflections,
terminate the line at both ends with its characteristic
impedance, and keep stub lengths off the main line as
short as possible. The slew-rate-limited MAX1480B/
MAX1480C/MAX1490B are more tolerant of imperfect
termination and stubs off the main line.
Layout Considerations
The MAX1480A/MAX1480B/MAX1480C/MAX1490A/
MAX1490B pinouts enable optimal PC board layout by
minimizing interconnect lengths and crossovers.
• For maximum isolation, the “isolation barrier” should
not be breached except by the MAX1480A/
MAX1480B/MAX1480C/MAX1490A/MAX1490B.
V_IN
5V
C1
C2
22µF
0.1µF
74HC86
OR EQUIVALENT
R1
DI
DI´
DRIVER INPUT
R2
RO
RECEIVER OUTPUT
MAX1490A/B
V_{CC1 1}
V_{CC2 2}
24 ^{AC1 (MAKE NO CONNECTION)}
23 AC2 (MAKE NO CONNECTION)
D1 ₃
D2 ₄
22 ^{ISO VCC1}
21 ^{ISO RO DRV}
120Ω
GND1 ₅
FS 6
20 ^A
19 ^B
A
B
SHIELD (OPTIONAL)
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)
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 MAX1490A/B FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
TRANSMISSION LINES.
LOGIC GROUND
Y
120Ω
D
DI
Z
B
120Ω
R
RO
A
Figure 14. Typical Full-Duplex RS-485/RS-422 Network
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