ISL81387IAZ Intersil, ISL81387IAZ Datasheet - Page 15

• Manufacturer Part Number: ISL81387IAZ
• Description: TXRX ESD RS-485/422 2PORT 20SSOP
• Manufacturer: Intersil
• Type: Transceiverr

Specifications of ISL81387IAZ

• Number Of Drivers/receivers: 2/2
• Protocol: RS422, RS485
• Voltage - Supply: 4.5 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 20-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Worst case receiver input currents are 20% lower than the 1
“unit load” (1mA) RS-485 limit, which translates to a 15kΩ
minimum input resistance.
These receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating), shorted together, or if the bus is
terminated but undriven (i.e., differential voltage collapses to
near zero due to termination). Failsafe with shorted, or
terminated and undriven inputs is accomplished by setting
the Rx upper switching point at -40mV, thereby ensuring that
the Rx recognizes a 0V differential as a high level.
All the Rx outputs are short circuit protected, and are
tri-statable via the active high RXEN pin, or when the IC is
shutdown (see Tables 2 and 3, and “Low Power Shutdown
(SHDN) Mode” on page 16). ISL41387 (QFN) receiver
outputs are also tri-statable via an active low RXEN input
(see “ISL41387 (QFN Package) Special Features” on
page 17 for more details).
For the ISL41387 (QFN), when using the active high RXEN
function, the RXEN pin may be left floating (internally pulled
high), or should be connected to V
If using the active low RXEN, then the RXEN pin must be
connected to GND.
TX FEATURES
The RS-485, RS-422 driver is a differential output device
that delivers at least 2.2V across a 54Ω load (RS-485), and
at least 2.5V across a 100Ω load (RS-422). Both levels
significantly exceed the standards requirements, and these
exceptional output voltages increase system noise immunity,
and/or allow for transmission over longer distances. The
drivers feature low propagation delay skew to maximize bit
widths, and to minimize EMI.
To allow multiple drivers on a bus, the RS-485 specification
requires that drivers survive worst case bus contentions
undamaged. The ISL81387, ISL41387 drivers meet this
requirement via driver output short circuit current limits, and
on-chip thermal shutdown circuitry. The output stages
incorporate current limiting circuitry that ensures that the
output current never exceeds the RS-485 specification, even
at the common mode voltage range extremes. In the event of
a major short circuit condition, devices also include a thermal
shutdown feature that disables the drivers whenever the die
temperature becomes excessive. This eliminates the power
dissipation, allowing the die to cool. The drivers automatically
re-enable after the die temperature drops about 15°. If the
contention persists, the thermal shutdown/re-enable cycle
repeats until the fault is cleared. Receivers stay operational
during thermal shutdown.
RS-485 multi-driver operation also requires drivers to include
tri-state functionality, so the port has a DEN pin to control this
function. If the driver is used in an RS-422 network, such that
driver tri-state isn’t required, then the DEN pin should connect
15
CC
through a 1kΩ resistor.
ISL81387, ISL41387
to V
the IC is in SHDN, or when the 5V power supply is off.
SPEED OPTIONS
The ISL81387 (SOIC/SSOP) features two speed options
that are user selectable via the SLEW pin: a high slew rate
setting optimized for 20Mbps data rates (Fast), and a slew
rate limited option for operation up to 460kbps (Med). The
ISL41387 (QFN) offers an additional, more slew rate limited,
option for data rates up to 115kbps (Slow). See “Data Rate,
Cables and Terminations” on page 15 and “RS-485 Slew
Rate Limited Data Rates” on page 17 for more information.
Receiver performance is the same for all three speed
options.
DATA RATE, CABLES AND TERMINATIONS
RS-485, RS-422 are intended for network lengths up to
4000’ (1220m), but the maximum system data rate
decreases as the transmission length increases. Devices
operating at the maximum data rate of 20Mbps are limited to
lengths of 20’ to 30’ (6m to 9m), while devices operating at or
below 115kbps can operate at the maximum length of 4000’
(1220m).
Higher data rates require faster edges, so both the
ISL81387, ISL41387 versions offer an edge rate capable of
20Mbps data rates. They both have a second option for
460kbps, but the ISL41387 also offers another, very slew
rate limited, edge rate to minimize problems at slow data
rates. Nevertheless, for the best jitter performance when
driving long cables, the faster speed settings may be
preferable, even at low data rates. See “RS-485 Slew Rate
Limited Data Rates” on page 17 for details.
Twisted pair is the cable of choice for RS-485, RS-422
networks. Twisted pair cables tend to pick up noise and
other electromagnetically induced voltages as common
mode signals, which are effectively rejected by the
differential receivers in these ICs.
The preferred cable connection technique is “daisy-chaining”,
where the cable runs from the connector of one device directly
to the connector of the next device, such that cable stub
lengths are negligible. A “backbone” structure, where stubs
run from the main backbone cable to each device’s connector,
is the next best choice, but care must be taken to ensure that
each stub is electrically “short”. See Table 4 for recommended
maximum stub lengths for each speed option.
CC
SPEED OPTION
through a 1kΩ resistor. Drivers are also tri-stated when
TABLE 4. RECOMMENDED STUB LENGTHS
SLOW
FAST
MED
MAXIMUM STUB LENGTH
350 to 500 (107 to 152)
100 to 150 (30.5 to 46)
1 to 3 (0.3 to 0.9)
ft (m)
November 21, 2007
FN6201.3