8
FN6073.4
July 26, 2007
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 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’, 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 surpasses the RS-422
specification of 4kΩ, and meets the RS-485 “Unit Load”
requirement of 12kΩ minimum.
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 rate supported by the
corresponding driver. ISL8489E/ISL8491E receiver outputs
are three-statable via the active low RE
input.
Driver 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 2V across a 100Ω load (RS-422). The drivers feature
low propagation delay skew to maximize bit width, and to
minimize EMI. ISL8489E/ISL8491E driver outputs are three-
statable via the active high DE input.
The ISL8488E/ISL8489E driver outputs are slew rate limited
to further reduce EMI, and to minimize reflections in
unterminated or improperly terminated networks. Data rates
on these slew rate limited versions are a maximum of
250kbps. Outputs of ISL8490E/ISL8491E drivers are not
limited, so faster output transition times allow data rates of at
least 10Mbps.
Data Rate, Cables, and Terminations
Twisted pair is the cable of choice for RS-485/RS-422
networks. Twisted pair cables tend to pick up noise and
other electromagnetically induced voltages as common
mode signals, which are effectively rejected by the
differential receivers in these ICs.
RS-485/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 10Mbps
are limited to lengths of a few hundred feet, while the
250kbps versions can operate at full data rates with lengths
in excess of 1000’.
Proper termination is imperative, when using the 10Mbps
devices, to minimize reflections. Short networks using the
250kbps versions need not be terminated, but, terminations
are recommended unless power dissipation is an overriding
concern. In point-to-point, or point-to-multipoint (single driver
on bus) networks, the main cable should be terminated in its
characteristic impedance (typically 120Ω) at the end farthest
from the driver. In multi-receiver applications, stubs
connecting receivers to the main cable should be kept as
short as possible. Multipoint (multi-driver) systems require
that the main cable be terminated in its characteristic
impedance at both ends. Stubs connecting a transceiver to
the main cable should be kept as short as possible.
Built-In Driver Overload Protection
As stated previously, the RS-485 specification requires that
drivers survive worst case bus contentions undamaged. The
ISL84xxE devices meet this requirement via driver output
short circuit current limits, and on-chip thermal shutdown
circuitry.
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-485 specification, even at the common
mode voltage range extremes. Additionally, these devices
utilize a foldback circuit which reduces the short circuit
current, and thus the power dissipation, whenever the
contending voltage exceeds either supply.
In the event of a major short circuit condition, ISL84xxE
devices also include a thermal shutdown feature that
disables the drivers whenever the die temperature becomes
excessive. This eliminates the power dissipation, allowing
the die to cool. The drivers automatically reenable after the
die temperature drops about 15°. If the contention persists,
the thermal shutdown/reenable cycle repeats until the fault is
cleared. Receivers stay operational during thermal
shutdown.
ESD Protection
All pins on these devices include class 3 Human Body Model
(HBM) ESD protection structures, but the RS-485 pins
(driver outputs and receiver inputs) incorporate advanced
structures allowing them to survive ESD events in excess of
ISL8488E, ISL8489E, ISL8490E, ISL8491E
