ISL3282EMRTEP Datasheet, PDF (4/8 Pages) Intersil Corporation – RS-485/RS-422 Receiver

ISL3282EMRTEP
Application Information
RS-485 and RS-422 are differential (balanced) data transmission
standards for use in 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.
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 Features
This device 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.
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.
Receiver inputs function with common mode voltages as great as
+9V/-7V outside the power supplies (i.e., +12V and -7V), making
them ideal for long networks where induced voltages, and ground
potential differences are realistic concerns.
The ISL3282EMRTEP includes 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 but undriven bus. Fail-safe with shorted inputs is
achieved by setting the Rx upper switching point to -50mV, thereby
ensuring that the Rx sees 0V differential as a high input level.
The receiver can easily support a 16Mbps data rate, and its
output is tri-statable via the active low RE input.
TABLE 2. V_IH, V_IL AND DATA RATE vs V_L FOR V_CC = 3.3V OR 5V
V_L
V_IH
V_IL
DATA RATE
(V)
(V)
(V)
(Mbps)
1.35
0.55
0.5
11
1.6
0.7
0.6
16
1.8
0.8
0.7
23
2.3
1
0.9
27
2.7
1.1
1
30
3.3
1.3
1.2
30
5.5 (i.e., V_CC)
2
1.8
24
V_CC = +3.3V TO 5V
V_CC = +2V
V_L
RO ^{VOH = 2V}
R_XD
ESD
DIODE
GND
V_IH = 1V
RE
RXEN
V_OH ≤ 2V
GND
ISL3282E
UART/PROCESSOR
FIGURE 3. USING V_L PIN TO ADJUST LOGIC LEVELS
Wide Supply Range
The ISL3282EMRTEP is designed to operate with a wide range
of supply voltages from 3.0V to 5.5V. This device meets the
RS-422 and RS-485 specifications over this full range.
Logic Supply (V_L Pin)
Note: Power-up V_CC before powering up the V_L supply.
The ISL3282EMRTEP includes a V_L pin that powers the logic
input (RE) and/or the Rx output. These pins interface with “logic”
devices such as UARTs, ASICs, and microcontrollers. Today, most
of these devices use power supplies significantly lower than 3.3V,
thus, a 3.3V output level from a 3.3V powered RS-485 IC might
seriously overdrive and damage the logic device input. Similarly,
the logic device’s low V_OH might not exceed the V_IH of a 3.3V or
5V powered RE input. Connecting the V_L pin to the power supply
of the logic device (as shown in Figure 3) limits the
ISL3282EMRTEP’s Rx output V_OH to V_L (see Figures 6 through
10), and reduces the RE input switching point to a value
compatible with the logic device’s output levels. Tailoring the
logic pin input switching point and output levels to the supply
voltage of the UART, ASIC, or microcontroller eliminates the need
for a level shifter/translator between the two ICs.
V_L can be anywhere from V_CC down to 1.35V, but the input
switching points may not provide enough noise margin when
V_L < 1.6V. Table 2 indicates typical V_IH, V_IL, and data rate values
for various V_L settings so the user can ascertain whether or not a
particular V_L voltage meets his/her needs.
The quiescent, RO unloaded, V_L supply current (I_L) is typically less
than 60µA for V_L ≤ 3.3V, as shown in Figure 5.
ESD Protection
All pins on the device include class 3 (>4kV) Human Body
Model (HBM) ESD protection structures, but the RS-485 pins
(receiver inputs) incorporate advanced structures allowing them
to survive ESD events in excess of ±16.5kV HBM and ±16.5kV
IEC61000. 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, and without degrading
the RS-485 common mode range of -7V to +12V. This built-in
4
FN7595.1
February 15, 2011