8
FN6050.8
March 14, 2016
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. To allow
for multipoint operation, the RS-485 spec requires that
drivers must handle bus contention without sustaining any
damage.
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 200mV, as required by the RS-422 and RS-485
specifications.
Receiver input resistance of 96k surpasses the RS-422
spec of 4k, and is eight times the RS-485 “Unit Load (UL)”
requirement of 12k 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 spec.
Receiver inputs function with common mode voltages as
great as 7V outside the power supplies (i.e., +12V and
-7V), making them ideal for long networks where induced
voltages are a realistic concern.
All the receivers include a “fail-safe if open” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating).
Receivers easily meet the data rates supported by the
corresponding driver, and receiver outputs are three-statable
via the active low RE
input.
Driver Features
The RS-485 and RS-422 driver is a differential output device
that delivers at least 1.5V across a 54 load (RS-485), and
at least 2V across a 100 load (RS-422). The drivers feature
low propagation delay skew to maximize bit width, and to
minimize EMI.
Driver outputs are three-statable via the active high DE
input.
The ISL8487 and ISL81483 driver outputs are slew rate
limited to minimize EMI, and to minimize reflections in
unterminated or improperly terminated networks. Data rate
on these slew rate limited versions is a maximum of
250kbps. ISL81487 drivers are not limited, so faster output
transition times allow data rates of at least 5Mbps.
Data Rate, Cables, and Terminations
RS-485 and RS-422 are intended for network lengths up to
4000’, but the maximum system data rate decreases as the
transmission length increases. Devices operating at 5Mbps
are limited to lengths less than a few hundred feet, while the
FIGURE 5A. TEST CIRCUIT
FIGURE 5B. MEASUREMENT POINTS
FIGURE 5. RECEIVER ENABLE AND DISABLE TIMES
Test Circuits and Waveforms (Continued)
1k
V
CC
GND
SW
(SHDN) for ISL8487 and ISL81483 only.
PARAMETER DE A SW
t
HZ
0 +1.5V GND
t
LZ
0-1.5VV
CC
t
ZH
(Note 6) 0 +1.5V GND
t
ZL
(Note 6) 0 -1.5V V
CC
t
ZH(SHDN)
(Note 7) 0 +1.5V GND
t
ZL(SHDN)
(Note 7) 0 -1.5V V
CC
SIGNAL
GENERATOR
R
RO
RE
A
B
15pF
RO
3V
0V
1.5V1.5V
V
OH
0V
1.5V
V
OH
- 0.5V
t
HZ
RO
V
CC
V
OL
1.5V
V
OL
+ 0.5V
t
LZ
RE
OUTPUT HIGH
OUTPUT LOW
t
ZL
, t
ZL(SHDN)
t
ZH
, t
ZH(SHDN)
NOTE 7
NOTE 7
NOTE 7
ISL8487, ISL81483, ISL81487
