ISL3158AE Datasheet, PDF (10/16 Pages) Intersil Corporation – ±16.5kV ESD (IEC61000-4-2) Protected, Large Output Swing, 5V, Full Fail-Safe, 1/8 Unit Load, RS-485/RS-422 Transceiver

ISL3158AE
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 long runs,
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
This device utilizes 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 @
V_OL = 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, this
product is known as a “one-eighth UL” transceiver, 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 driver features
low propagation delay skew to maximize bit width, and to
minimize EMI, and all drivers are three-stateable via the
active high DE input.
Outputs of the ISL3158AE driver is not limited, so faster
output transition times allow data rates of at least 10Mbps
High V_OD Improves Noise Immunity and Flexibility
The ISL3158AE driver design delivers larger differential
output voltages (V_OD) than the RS-485 standard requires, or
than most RS-485 transmitters can deliver. The minimum
±2.4V V_OD guarantees at least ±900mV more noise
immunity than networks built using standard 1.5V V_OD
transmitters.
Another advantage of the large V_OD is the ability to drive
more than two bus terminations, which allows for utilizing the
ISL3158AE in “star” and other multi-terminated,
“non-standard” network topologies. Figure 8, details the
transmitter’s V_OD vs I_OUT 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 V_OD
into two terminations, but the ISL3158AE 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 ISL3158AE
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 ~3.4V. This gives the
processor/ASIC a chance to stabilize and drive the RS-485
control lines to the proper states.
V_CC
5.0
2.5
A/Y
0
RO
3.5V
3.3V
ISL3158AE
ISL3158AE
DE, DI = V_CC
RE = GND 5.0
2.5
0
R_L = 1kΩ
5.0
R_L = 1kΩ
2.5
0
TIME (40µs/DIV)
FIGURE 7. HOT PLUG PERFORMANCE (ISL3158AE) vs
ISL83088E WITHOUT HOT PLUG CIRCUITRY
10
FN6886.0
April 3, 2009