isl3179e Intersil Corporation, isl3179e Datasheet - Page 9

• Manufacturer Part Number: isl3179e
• Description: ?15kv Esd Protected, +125?c, 40mbps, 3.3v, Full Fail-safe, Rs-485/rs-422 Transceivers
• Manufacturer: Intersil Corporation

Data Rate, Cables, and Terminations
RS-485/422 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 multi-receiver applications, stubs
connecting receivers to the main cable should be kept as
short as possible. Multipoint (multi-driver) 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 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.
9
ISL3179E
Built-In Driver Overload Protection
As stated previously, the RS-485 spec 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 spec, 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
to a 50nA trickle. It enters shutdown whenever the receiver
and driver are simultaneously disabled (RE = V
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 7, 8,
9, 10 and 11, at the end of the “Electrical Specification” table
on page 5, for more information.
CC
August 16, 2007
and
FN6365.1
CC