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ISL3150EIBZ Datasheet(PDF) 12 Page - Intersil Corporation

Part No. ISL3150EIBZ
Description  ±16.5kV ESD, Large Output Swing, 5V, Full Fail-Safe, 1/8 Unit Load, RS-485/RS-422 Transceivers
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

ISL3150EIBZ Datasheet(HTML) 12 Page - Intersil Corporation

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12
FN6363.2
July 30, 2009
Application Information
RS-485 and RS-422 are differential (balanced) data
transmission standards used for 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
specification 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 (Rx) Features
These devices utilize a differential input receiver for
maximum noise immunity and common mode rejection.
Input sensitivity is better than ±200mV, as required by
the RS-422 and RS-485 specifications.
Rx outputs feature high drive levels (typically 28mA @
VOL = 1V) to ease the design of optically coupled
isolated interfaces.
Receiver input resistance of 96kΩ surpasses the RS-422
specification of 4kΩ, and is eight times the RS-485 “Unit
Load (UL)” requirement of 12kΩ minimum. Thus, these
products are known as “one-eighth UL” transceivers, and
there can be up to 256 of these devices on a network
while still complying with the RS-485 loading
specification.
Rx 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.
All the receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver
inputs are unconnected (floating), shorted together, or
connected to a terminated bus with all the transmitters
disabled.
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 (Tx) Features
The RS-485/RS-422 driver is a differential output device
that delivers at least 2.4V across a 54Ω load (RS-485),
and at least 2.8V across a 100Ω load (RS-422). The
drivers feature low propagation delay skew to maximize
bit width, and to minimize EMI, and all drivers are
three-statable via the active high DE input.
The 115kbps and 1Mbps driver outputs are slew rate
limited to minimize EMI, and to minimize reflections in
unterminated or improperly terminated networks.
Outputs of the ISL3156E and ISL3158E drivers are not
limited, so faster output transition times allow data rates
of at least 20Mbps.
HIGH VOD IMPROVES NOISE IMMUNITY AND
FLEXIBILITY
The ISL315xE driver design delivers larger differential
output voltages (VOD) than the RS-485 standard
requires, or than most RS-485 transmitters can deliver.
The minimum ±2.4V VOD guarantees at least ±900mV
more noise immunity than networks built using standard
1.5V VOD transmitters.
Another advantage of the large VOD is the ability to drive
more than two bus terminations, which allows for
utilizing the ISL315xE in “star” and other
multi-terminated, “nonstandard” network topologies.
Figure 8, details the transmitter’s VOD vs IOUT
characteristic, and includes load lines for six (20Ω) and
eight (15Ω) 120Ω terminations. The figure shows that
the driver typically delivers 1.65/1.5V into 6/8
terminations, even at the worst case temperature of
+85°C. The RS-485 standard requires a minimum 1.5V
VOD into two terminations, but the ISL315xE delivers
RS-485 voltage levels with 3x to 4x the number of
terminations.
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 ISL315xE devices incorporate a
“Hot Plug” function. Circuitry monitoring VCC ensures
that, during power-up and power-down, the Tx and Rx
outputs remain disabled, regardless of the state of DE
and RE, if VCC 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.
TIME (40µs/DIV)
VCC
2.5
5.0
2.5
5.0
RL = 1kΩ
RO
0
2.5
5.0
0
0
A/Y
RL = 1kΩ
3.3V
3.5V
ISL315xE
ISL315xE
RE = GND
DE, DI = VCC
FIGURE 7. HOT PLUG PERFORMANCE (ISL315xE) vs
ISL83088E WITHOUT HOT PLUG
CIRCUITRY
ISL3150E, ISL3152E, ISL3153E, ISL3155E, ISL3156E, ISL3158E


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