ISL32483E Datasheet, PDF (11/17 Pages) Intersil Corporation – Fault Protected, Extended CMR, RS-485/RS-422 Transceivers

ISL32483E, ISL32485E
Intersil’s unique cable invert function is superior to that found on
competing devices, because the Rx full fail-safe function is
maintained, even when the Rx polarity is reversed. Competitor
devices implement the Rx invert function simply by inverting the
Rx output. This means that with the Rx inputs floating or shorted
together, the Rx appropriately delivers a logic 1 in normal
polarity, but outputs a logic low when the IC is operated in the
inverted mode. Intersil’s innovative Rx design guarantees that,
with the Rx inputs floating or shorted together (V_ID=0V), the Rx
output remains high, regardless of the state of the invert pins.
The full duplex ISL32483E includes two invert pins that allow for
separate control of the Rx and Tx polarities. If only the Rx cable is
miswired, then only the RINV pin need be driven to a logic 1. If
the Tx cable is miswired, then DINV must be connected to a logic
high. The half-duplex version has only one logic pin (INV) that,
when high, switches the polarity of both the Tx and the Rx blocks.
High V_OD Improves Noise Immunity and
Flexibility
The ISL3248xE driver design delivers larger differential output
voltages (V_OD) than the RS-485 standard requires or than most
RS-485 transmitters can deliver. The typical ±2.5V V_OD provides
more noise immunity than networks built using many other
transceivers.
Another advantage of the large V_OD is the ability to drive more
than two bus terminations, which allows for utilizing the
ISL3248xE in “star” and other multi-terminated, nonstandard
network topologies.
Figure 10 details the transmitter’s V_OD versus I_OUT characteristic,
and includes load lines for four (30Ω) and six (20Ω) 120Ω
terminations. The figure shows that the driver typically delivers
±1.3V into six terminations, and the “Electrical Specifications” on
page 5 guarantee a V_OD of ±0.8V at 21Ω over the full
temperature range. The RS-485 standard requires a minimum
1.5V V_OD into two terminations, but the ISL3248xE delivers
RS-485 voltage levels with 2x to 3x the number of terminations.
Hot Plug Function
When a piece of equipment powers up, there is a period of time
in which 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 a bus, a
driver activating prematurely during power-up may crash the bus.
To avoid this scenario, the ISL3248xE devices 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 ≈3.5V. This
gives the processor or ASIC a chance to stabilize and drive the
RS-485 control lines to the proper states. Figure 9 illustrates the
power-up and power-down performance of the ISL3248xE
compared to an RS-485 IC without the Hot Plug feature.
V_CC
3.5V
2.8V
5.0
2.5
A/Y
0
ISL83088E
ISL3248XE
RO
ISL3248XE
DE, DI = V_CC
RE = GND
5.0
2.5
0
RL = 1kΩ
5.0
RL = 1kΩ
2.5
0
TIME (40µs/DIV)
FIGURE 9. HOT PLUG PERFORMANCE (ISL3248XE) vs ISL83088E
WITHOUT HOT PLUG CIRCUITRY
ESD Protection
All pins on the ISL3248xE devices include Class 3 (>8kV)
Human Body Model (HBM) ESD protection structures that are
good enough to survive ESD events commonly seen during
manufacturing. Even so, the RS-485 pins (driver outputs and
receiver inputs) incorporate more advanced structures that
allow them to survive ESD events in excess of ±16.5kV HBM
(±15kV for full-duplex version). The RS-485 pins are particularly
vulnerable to ESD strikes, 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. The new ESD
structures protect the device whether or not it is powered up,
and without interfering with the exceptional ±25V CMR. This
built-in ESD protection minimizes 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/RS-422 are intended for network lengths up to 4000 feet,
but the maximum system data rate decreases as the transmission
length increases. These 1Mbps versions can operate at full data
rates with lengths up to 800 feet (244m). Jitter is the limiting
parameter at this data rate, so employing encoded data streams
(e.g., Manchester coded or Return-to-Zero) may allow increased
transmission distances.
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.
11
FN7785.0
January 18, 2011