ISL31480E_10 Datasheet, PDF (15/24 Pages) Intersil Corporation – Fault Protected, Extended CMR, RS-485/RS-422 Transceivers with Cable Invert

ISL31480E, ISL31483E, ISL31485E, ISL31486E
The ISL31483E, ISL31485E, and ISL31486E simplify this
task by including cable invert pins (INV, DINV, RINV) that
allow the technician to invert the polarity of the Rx input
and/or the Tx output pins simply by moving a jumper to
change the state of the invert pin(s). When the invert
pin(s) is low, the IC operates like any standard RS-485
transceiver and the bus pins have their normal polarity
definition of A and Y being noninverting, and B and Z
being inverting. With the invert pin high, the
corresponding bus pins reverse their polarity, so B and Z
are now noninverting and A and Y become inverting.
Intersil’s unique cable invert function is superior to that
found on competing devices because the Rx full failsafe
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 ISL31483E 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 two
half duplex versions have only one logic pin (INV) that,
when high, switches the polarity of both the Tx and the
Rx blocks.
Logic Supply (V_L Pin)
Note: Power up V_CC before powering up the V_L supply,
and keep V_L ≤ V_CC.
The ISL31480E and ISL31486E include a V_L pin that
powers the logic inputs (Tx input and control pins) and the
Rx output. These pins interface with “logic” devices such as
UARTs, ASICs, and µcontrollers, and today many of these
devices use power supplies significantly lower than 5V.
Thus, a 5V output level from this transceiver IC might
seriously overdrive and damage the logic device input (see
Figure 7). Similarly, the logic device’s low V_OH might not
exceed the V_IH of a 5V powered transceiver input.
Connecting the V_L pin to the power supply of the logic
device - as shown in Figure 7 - limits the ISL3148xE’s RO
pin V_OH to the V_L voltage, and reduces the Tx and control
input switching points to values compatible with the logic
device output levels. Tailoring the logic pin input switching
points and output levels to the supply voltage of the UART,
ASIC, or µcontroller eliminates the need for a level
shifter/translator between the two ICs.
V_L can be anywhere from V_CC down to 1.62V, and the
transceivers easily operate at the 1Mbps data rate over
this range as long as the VCM doesn’t exceed ±15V.
Table 2 indicates typical V_IH and V_IL values for various
V_L voltages so the user can ascertain whether or not a
particular V_L voltage meets his/her needs.
V_CC = +5V
V_CC = +1.8V
GND
RO ^{VOH = 5V}
R_XD
ESD
DIODE
V_IH ≥ 2V
DI
T_XD
V_OH ≈ 1.8V
GND
ISL31483E
V_CC = +5V
UART/PROCESSOR
V_CC = +1.8V
GND
V_L
RO ^{VOH = 1.8V}
R_XD
V_IH = 1.1V
DI
T_XD
V_OH ≈ 1.8V
ESD
DIODE
GND
ISL31480E
UART/PROCESSOR
FIGURE 7. USING V_L PIN TO ADJUST LOGIC LEVELS
TABLE 2. V_IH AND V_IL vs. V_L FOR V_CC = 5V
V_L (V)
1.6
V_IH (V)
1.0
V_IL (V)
0.6
1.8
1.1
0.7
2.3
1.3
0.9
2.7
1.4
1.1
3.3
1.6
1.3
The V_L supply current (I_L) is typically less than 6µA. All
of the DC V_L current is due to current through the DE
input internal pull-up resistor when the pin is driven to
the low input state.
Transceiver logic inputs that are externally tied high in an
application should use the V_L supply for the high voltage
level to minimize input currents. Except for DI, all logic
inputs have 800kΩ pull-up (DE) or pull-down (all other
pins) resistors, so connecting an input to the lower voltage
V_L supply minimizes current. The DE pull-up internally
connects to V_L, so connecting the DE pin to V_CC induces an
input current of (V_CC - V_L)/800kΩ.
High V_OD Improves Noise Immunity and
Flexibility
The ISL3148xE 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
15
FN7638.0
June 25, 2010