9
FN6048.10
September 3, 2015
Data Rate, Cables, and Terminations
RS485, RS-422 are intended for network lengths up to 4000
feet, but the maximum system data rate decreases as the
transmission length increases. Devices operating at 10Mbps
are limited to lengths less than 100 feet, while the 250kbps
versions can operate at full data rates with lengths in excess
of 1000 feet.
Twisted pair is the cable of choice for RS485, 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.
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 120) 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
ISL848xE 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, ISL848xE
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°. If the contention persists,
the thermal shutdown/re-enable cycle repeats until the fault
is cleared. Receivers stay operational during thermal
shutdown.
Low Power Shutdown Mode (ISL8483E Only)
These CMOS transceivers all use a fraction of the power
required by their bipolar counterparts, but the ISL8483E
includes a shutdown feature that reduces the already low
quiescent I
CC
to a 1nA trickle. The ISL8483E enters
shutdown whenever the receiver and driver are
simultaneously disabled (RE
=V
CC
and DE = GND) for a
period of at least 600ns. Disabling both the driver and the
receiver for less than 50ns guarantees that the ISL8483E
will not enter shutdown.
Note that receiver and driver enable times increase when
the ISL8483E enables from shutdown. Refer to Notes 5-8,
on page 6, at the end of the “Electrical Specifications” table,
for more information.
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) 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.
Human Body Model Testing
As the name implies, this test method emulates the ESD
event delivered to an IC during human handling. The tester
delivers the charge stored on a 100pF capacitor through a
1.5k current limiting resistor into the pin under test. The
HBM method determines an IC’s ability to withstand the ESD
events typically present during handling and manufacturing.
The RS-485 pin survivability on this high ESD family has
been characterized to be in excess of
15kV, for discharges
to GND.
ISL8483E, ISL8485E
