ISL31470E_10 Datasheet, PDF (12/21 Pages) Intersil Corporation – Fault Protected, Extended Common Mode Range, RS-485/RS-422 Transceivers

ISL31470E, ISL31472E, ISL31475E, ISL31478E
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards used for 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
specification 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’, thus the wide CMR is
necessary to handle ground potential differences, as well
as voltages induced in the cable by external fields.
The ISL31470E, ISL31472E, ISL31475E, ISL31478E is a
family of ruggedized RS-485 transceivers that improves
on the RS-485 basic requirements, and therefore
increases system reliability. The CMR increases to ±15V,
while the RS-485 bus pins (receiver inputs and driver
outputs) include fault protection against voltages and
transients up to ±60V. Additionally, larger than required
differential output voltages (V_OD) increase noise
immunity.
Receiver (Rx) 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 (load) current surpasses the RS-422
specification of 3mA, and is four times lower than the
RS-485 “Unit Load (UL)” requirement of 1mA maximum.
Thus, these products are known as “one-quarter UL”
transceivers, and there can be up to 128 of these devices
on a network while still complying with the RS-485
loading specification.
The Rx functions with common mode voltages as great
as ±15V, making them ideal for industrial, or long
networks where induced voltages are a realistic concern.
All the receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver
inputs are unconnected (floating), shorted together, or
connected to a terminated bus with all the transmitters
disabled (i.e., an idle bus).
Rx outputs feature high drive levels (typically 22mA @
V_OL = 1V) to ease the design of optically-coupled
isolated interfaces.
Receivers easily meet the data rates supported by the
corresponding driver, and all receiver outputs are
three-statable via the active low RE input.
The Rx in the 250kbps and 1Mbps versions include noise
filtering circuitry to reject high frequency signals. The
1Mbps version typically rejects pulses narrower than
50ns (equivalent to 20Mbps), while the 250kbps Rx
rejects pulses below 150ns (6.7Mbps).
Driver (Tx) Features
The RS-485/RS-422 driver is a differential output device
that delivers at least 1.5V across a 54Ω load (RS-485),
and at least 2.4V across a 100Ω load (RS-422). The
drivers feature low propagation delay skew to maximize
bit width, and to minimize EMI, and all drivers are
three-statable via the active high DE input.
The 250kbps and 1Mbps driver outputs are slew rate
limited to minimize EMI, and to minimize reflections in
unterminated or improperly terminated networks.
Outputs of the ISL31478E drivers are not limited, thus,
faster output transition times allow data rates of at least
15Mbps.
High Overvoltage (Fault) Protection
Increases ruggedness
The ±60V (referenced to the IC GND) fault protection on
the RS-485 pins, makes these transceivers some of the
most rugged on the market. This level of protection
makes the ISL31470E, ISL31472E, ISL31475E,
ISL31478E perfect for applications where power
(e.g., 24V and 48V supplies) must be routed in the
conduit with the data lines, or for outdoor applications
where large transients are likely to occur. When power is
routed with the data lines, even a momentary short
between the supply and data lines will destroy an
unprotected device. The ±60V fault levels of this family
are at least five times higher than the levels specified
for standard RS-485 ICs. The ISL31470E, ISL31472E,
ISL31475E, ISL31478E protection is active whether the
Tx is enabled or disabled, and even if the IC is powered
down.
If transients or voltages (including overshoots and
ringing) greater than ±60V are possible, then additional
external protection is required.
Wide Common Mode Voltage (CMV)
Tolerance Improves Operating Range
RS-485 networks operating in industrial complexes, or
over long distances, are susceptible to large CMV
variations. Either of these operating environments may
suffer from large node-to-node ground potential
differences, or CMV pickup from external electromagnetic
sources, and devices with only the minimum required
+12V to -7V CMR may malfunction. The ISL31470E,
ISL31472E, ISL31475E, ISL31478E’s extended ±15V
CMR allows for operation in environments that would
overwhelm lesser transceivers. Additionally, the Rx will
not phase invert (erroneously change state) even with
CMVs of ±40V, or differential voltages as large as 40V.
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FN7639.0
June 17, 2010