ISL3150E, ISL3152E, ISL3153E, ISL3155E, ISL3156E, ISL3158E
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
ISL3156Eand ISL3158E drivers are not limited, so faster
output transition times allow data rates of at least 20Mbps
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 spec requires that
drivers must handle bus contention without sustaining any
damage.
High V
Improves Noise Immunity and Flexibility
OD
The ISL315XE driver design delivers larger differential
output voltages (V ) than the RS-485 standard requires, or
OD
than most RS-485 transmitters can deliver. The minimum
±2.4V V
guarantees at least ±900mV more noise
OD
immunity than networks built using standard 1.5V V
transmitters.
OD
Another advantage of the large V
is the ability to drive
OD
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.
more than two bus terminations, which allows for utilizing the
ISL315XE in “star” and other multi-terminated, “non-
standard” network topologies. Figure 8, details the
transmitter’s V
vs. I characteristic, and includes load
OD
OUT
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
into two terminations, but the ISL315XE delivers RS-485
voltage levels with 3X to 4X the number of terminations.
Receiver (Rx) Features
OD
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.
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
Rx outputs feature high drive levels - typically 28mA @
V
= 1V (to ease the design of optically coupled isolated
OL
interfaces).
Receiver input resistance of 96kΩ surpasses the RS-422
spec 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 spec.
V
ensures that, during power up and power down, the Tx
CC
and Rx outputs remain disabled, regardless of the state of DE
and RE, if V is less than ~3.4V. This gives the
CC
processor/ASIC a chance to stabilize and drive the RS-485
control lines to the proper states.
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.
DE, DI = V
CC
RE = GND
5
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.
3.5V
3.3V
2.5
V
CC
0
5
2.5
0
R
R
= 1kΩ
= 1kΩ
L
L
Receivers easily meet the data rates supported by the
corresponding driver, and all receiver outputs are three-
statable via the active low RE input.
A/Y
ISL315XE
ISL315XE
Driver (Tx) Features
5
The RS-485/422 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.
2.5
RO
0
TIME (40μs/DIV)
FIGURE 7. HOT PLUG PERFORMANCE (ISL315XE) vs
ISL83088E WITHOUT HOT PLUG CIRCUITRY
FN6363.0
December 14, 2006
11
INTERSIL [ Intersil ]
