ISL8487E, ISL81487L, ISL81487E
All the receivers include a “fail-safe if open” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating).
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-485 spec, 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.
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
In the event of a major short circuit condition, these 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 re-enable after the die
temperature drops about 15 degrees. If the contention
persists, the thermal shutdown/re-enable cycle repeats until
the fault is cleared. Receivers stay operational during
thermal shutdown.
The RS-485/422 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.
Driver outputs are three-statable via the active high DE
input.
The ISL8487E and ISL81487L 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. ISL81487E drivers are not limited, so faster output
transition times allow data rates of at least 5Mbps.
Low Power Shutdown Mode (Excluding
ISL81487E)
These CMOS transceivers all use a fraction of the power
required by their bipolar counterparts, but the ISL8487E
and ISL81487L include a shutdown feature that reduces
the already low quiescent I
to a 500nA trickle. They
CC
enter shutdown whenever the receiver and driver are
simultaneously disabled (RE = V and DE = GND) for a
Data Rate, Cables, and Terminations
CC
RS-485/422 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
250kbps versions can operate at full data rates with lengths
in excess of 1000’.
period of at least 600ns. Disabling both the driver and the
receiver for less than 50ns guarantees that shutdown is not
entered.
Note that receiver and driver enable times increase when
enabling from shutdown. Refer to Notes 5-9, at the end of
the Electrical Specification table, for more information.
Twisted pair is the cable of choice for RS-485/422 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.
ESD Protection
All pins on these interface devices include class 3 Human
Body Model (HBM) ESD protection structures, but the
RS-485 pins (driver outputs and receiver inputs)
To minimize reflections, proper termination is imperative
when using the 5Mbps device. Short networks using the
250kbps versions need not be terminated, but, terminations
are recommended unless power dissipation is an overriding
concern.
incorporate advanced structures allowing them to survive
ESD events in excess of ±15kV HBM. The RS-485 pins are
particularly vulnerable to ESD damage because they
typically connect to an exposed port on the exterior of the
finished product. Simply touching the port pins, or
connecting a cable, can cause an ESD event that might
destroy unprotected ICs. These new ESD structures
protect the device whether or not it is powered up, protect
without allowing any latchup mechanism to activate, and
without degrading the RS-485 common mode range of -7V
to +12V. This built-in ESD protection eliminates the need
for board level protection structures (e.g., transient
suppression diodes), and the associated, undesirable
capacitive load they present.
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 spec requires that drivers
survive worst case bus contentions undamaged. These
devices meet this requirement via driver output short circuit
current limits, and on-chip thermal shutdown circuitry.
FN6051.6
9
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