ISL3280E Datasheet, PDF (8/15 Pages) Intersil Corporation – ±16.5kV ESD Protected, +125°C, 3.0V to 5.5V, SOT-23/TDFN Packaged, 20Mbps Full Fail-safe, Low Power, RS-485/RS-422 Receivers

ISL3280E, ISL3281E, ISL3282E, ISL3283E, ISL3284E, ISL3285E
Test Circuits and Waveforms
V_CC ^RE
GND
RE
B
RO
A
R
15pF
SIGNAL
GENERATORS
B
0V
A
+1V
0V
-1V
t_PLH
t_PHL
V_CC OR V_L
RO
50%
50%
0V
FIGURE 1A. TEST CIRCUIT
FIGURE 1B. MEASUREMENT POINTS
FIGURE 1. RECEIVER PROPAGATION DELAY AND DATA RATE
SIGNAL
GENERATOR
RE OR RE
GND
B
RO
AR
1kΩ
15pF
V_CC OR V_L
SW ^GND
PARAMETER
t_HZ
t_LZ
t_ZH
t_ZL
A
+1.5V
-1.5V
+1.5V
-1.5V
SW
GND
V_CC OR V_L
GND
V_CC OR V_L
RE
(INVERT FOR RE)
RO
RO
1.5V
3V
1.5V
0V
t_ZH
t_HZ
OUTPUT HIGH
50%
V_OH - 0.25V ^VOH
0V
t_ZL
t_LZ
V_CC OR V_L
50%
OUTPUT LOW
V_OL + 0.25V _VOL
FIGURE 2A. TEST CIRCUIT
FIGURE 2B. MEASUREMENT POINTS
FIGURE 2. RECEIVER ENABLE AND DISABLE TIMES (EXCEPT ISL3280E AND ISL3284E)
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards for use in long haul or noisy
environments. RS-422 is a subset of RS-485, so RS-485
transceivers are also RS-422 compliant. RS-422 is a
point-to-multipoint (multidrop) standard, which allows only one
driver and up to 10 (assuming one unit load devices) receivers
on each bus. RS-485 is a true multipoint standard, which allows
up to 32 one unit load devices (any combination of drivers and
receivers) on each bus.
Another important advantage of RS-485 is the extended
common mode range (CMR), which specifies that the driver
outputs and receiver inputs withstand signals that range from
+12V to -7V. RS-422 and RS-485 are intended for runs as
long as 4000’, so the wide CMR is necessary to handle
ground potential differences, as well as voltages induced in
the cable by external fields.
Receiver Features
These devices utilize a differential input receiver for maximum
noise immunity and common mode rejection. Input sensitivity
is better than ±200mV, as required by the RS-422 and RS-485
specifications.
Receiver input resistance of 96kΩ surpasses the RS-422
specification of 4kΩ and is eight times the RS-485 “Unit
Load (UL)” requirement of 12kΩ minimum. Thus, these
products are known as “one-eighth UL” transceivers and
there can be up to 256 of these devices on a network while
still complying with the RS-485 loading specification.
Receiver inputs function with common mode voltages as great
as +9V/-7V outside the power supplies (i.e., +12V and -7V),
making them ideal for long networks where induced voltages,
and ground potential differences are realistic concerns.
8
FN6543.2
October 18, 2007