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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
The ISL4485E utilizes 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 impedance surpasses the RS-422 spec 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.
The receiver includes a “fail-safe if open” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating). The output is three-statable via
the active low RE
input, and the receiver easily meets the
20Mbps data rate.
Driver Features
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 ISL4485E driver
features low propagation delay skew to maximize bit width,
and to minimize EMI, and the outputs are three-statable via
the active high DE input.
Outputs of ISL4485E drivers are not slew rate limited, so
faster output transition times allow data rates up to 20Mbps.
Data Rate, Cables, and Terminations
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.
RS-485/422 are intended for network lengths up to 4000',
but the maximum transmission length decreases as the data
rate increases. According to guidelines in the RS-422
specification, a 20Mbps network should be limited to less
than 50' of 24 AWG twisted pair. Nevertheless, the
ISL4485E's large differential voltage swing, fast transition
times, and high drive-current output stages allow operation
at 20Mbps in RS-485/422 networks as long as 450'. Figure 7
details ISL4485E operation at 20Mbps driving 300' of CAT 5
cable terminated in 120Ω at the driver and the receiver (i.e.,
double terminated). The acceptance criteria for this test was
the ability of the driver to deliver a 1.5V differential signal to
the receiver at the end of the cable (i.e., |A-B| ≥ 1.5V). If a
more liberal acceptance criteria is used, the distance can be
further extended. For example, Figure 8 illustrates the
performance in the same configuration, but with a cable
length of 450', and an acceptance criteria of no more than
6dB attenuation across the cable (i.e., |A-B| = |Y-Z|/2).
Driver differential output voltage decreases with increasing
differential load capacitance, so maintaining a 1.5V
differential output requires a data rate reduction, as shown in
Figure 9.
To minimize reflections, proper termination is imperative
when using this 20Mbps device. 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 (preferably less
than 12 inches). Multipoint (multi-driver) systems require that
the main cable be terminated in its characteristic impedance
at both ends. Again, 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. The
ISL4485E device meets 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 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.
In the event of a major short circuit condition, this device
also includes 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
ISL4485E
