ISL3179E_15 Datasheet, PDF (13/18 Pages) Intersil Corporation – High ESD Protected, +125°C, 40Mbps, 3.3V, Full Fail-Safe, RS-485/RS-422 Transceivers

ISL3179E, ISL3180E
“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 ~2.4V. This
gives the processor/ASIC a chance to stabilize and drive the RS-485
control lines to the proper states.
V_CC
4
2
A/Y
0
RO
2.5V
2.3V
ISL3179E
ISL3179E
DE, DI = V_CC
RE = GND
4
2
0
R_L = 1kΩ
4
R_L = 1kΩ
2
0
TIME (40µs/DIV)
FIGURE 21. HOT PLUG PERFORMANCE (ISL3179E) vs ISL83485
WITHOUT HOT PLUG CIRCUITRY
Data Rate, Cables, and Terminations
RS-485/RS-422 are intended for network lengths up to 4000’,
but the maximum system data rate decreases as the
transmission length increases. Devices operating at 40Mbps are
limited to lengths less than 100’.
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 receiver in this IC.
Proper termination is imperative to minimize reflections. In point-
to-point, or point-to-multipoint (single driver on bus) networks,
the main cable should be terminated in its characteristic
impedance (typically 120Ω) at the end farthest from the driver. In
multireceiver applications, stubs connecting receivers to the
main cable should be kept as short as possible. Multipoint
(multidriver) systems require that the main cable be terminated
in its characteristic impedance at both ends. Stubs connecting a
transceiver to the main cable should be kept as short as possible.
The ISL3179E and ISL3180E may also be used at slower data
rates over longer cables, but there are some limitations. The Rx is
optimized for high speed operation, so its output may glitch if the
Rx input differential transition times are too slow. Keeping the
transition times below 500ns, which equates to the Tx driving a
1000’ (305m) CAT 5 cable, yields excellent performance over the
full operating temperature range.
Built-in Driver Overload Protection
As stated previously, the RS-485 specification requires that
drivers survive worst case bus contentions undamaged. These
transmitters meet this requirement via driver output short circuit
current limits, and on-chip thermal shutdown circuitry.
The driver output stages incorporate short-circuit current limiting
circuitry, which 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, the device
also includes 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 reenable after the die temperature drops about
+15°C. If the contention persists, the thermal shutdown/reenable
cycle repeats until the fault is cleared. Receivers stay operational
during thermal shutdown.
Low Power Shutdown Mode
This BiCMOS transceiver uses a fraction of the power required by
their bipolar counterparts, but it also includes a shutdown
feature that reduces the already low quiescent I_CC to a 50nA
trickle. It enters shutdown whenever the receiver and driver are
simultaneously disabled (RE = V_CC and DE = GND) for a period of
at least 600ns. Disabling both the driver and the receiver for less
than 60ns guarantees that the transceiver will not enter
shutdown.
Note that receiver and driver enable times increase when the
transceiver enables from shutdown. Refer to Notes 10, 11, 12,
13 and 14, at the end of the “Electrical Specifications” table on
page 7, for more information.
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FN6365.5
August 25, 2015