
11
FN6085.7
November 4, 2005
Receivers easily meet the data rates supported by the 
corresponding driver, and all receiver outputs are three-
statable via the active low RE input. 
Driver Features
The RS-485/422 driver is a differential output device that 
delivers at least 1.5V across a 54
 load (RS-485), and at 
least 2V across a 100
 load (RS-422). The drivers feature 
low propagation delay skew to maximize bit width, and to 
minimize EMI.
All drivers are three-statable via the active high DE input. 
The 115kbps and 500kbps driver outputs are slew rate 
limited to minimize EMI, and to minimize reflections in 
unterminated or improperly terminated networks. Outputs of 
the ISL83086E, ISL83088E drivers are not limited, so faster 
output transition times allow data rates of at least 10Mbps.
Hot Plug Function
When a piece of equipment powers up, there is a period of 
time where the processor or ASIC driving the RS-485 control 
lines (DE, RE) is unable to ensure that the RS-485 Tx and 
Rx outputs are kept disabled. If the equipment is connected 
to the bus, a driver activating prematurely during power up 
may crash the bus. To avoid this scenario, the ISL83080, 
ISL83082, ISL83083, ISL83085 versions incorporate a “Hot 
Plug” function. Circuitry monitoring V
CC 
ensures that, during 
power up and power down, the Tx and Rx outputs remain 
disabled, regardless of the state of DE and RE, if V
CC
 is less 
than ~3.4V. This gives the processor/ASIC a chance to stabilize 
and drive the RS-485 control lines to the proper states.
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 
±
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.
Data Rate, Cables, and Terminations 
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 10Mbps 
are limited to lengths less than 100’, while the 115kbps 
versions can operate at full data rates with lengths of several 
thousand feet.
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.
Proper termination is imperative, when using the 10Mbps 
devices, to minimize reflections. Short networks using the 
115kbps 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 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.
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, 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.
FIGURE 7.
 HOT PLUG PERFORMANCE (ISL83080E) vs 
DEVICE WITHOUT HOT PLUG CIRCUITRY 
(ISL83086E)
TIME (40μs/DIV)
V
CC
R
D
2.5
5
2.5
5
V
C
R
L
 = 1k
RO
0
2.5
5
0
0
A
/
Y
R
L
 = 1k
ISL83080E
ISL83080E
3.2V
3.4V
DI = V
CC
ISL83080E, ISL83082E, ISL83083E, ISL83085E, ISL83086E, ISL83088E