ISL41334 INTERSIL [Intersil Corporation], ISL41334 Datasheet - Page 17

• Manufacturer Part Number: ISL41334
• Manufacturer: INTERSIL [Intersil Corporation]

Data Rate, Cables, and Terminations
RS-485/422 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-
30’ (6-9m), 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
ISLX1334 versions offer an edge rate capable of 20Mbps
data rates. The ISL41334 also offers two slew rate limited
edge rates to minimize problems at slower 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 the “RS-485 Slew Rate Limited
Data Rates” section for details.
Twisted pair is the cable of choice for RS-485/422 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.
Proper termination is imperative to minimize reflections
when using the 20Mbps speed option. Short networks using
the medium and slow speed options need not be terminated,
but terminations are recommended unless power dissipation
is an overriding concern. Note that the RS-485 spec allows a
maximum of two terminations on a network, otherwise the Tx
output voltage may not meet the required V
In point-to-point, or point-to-multipoint (RS-422) 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, but definitely shorter than the limits shown in Table
4. Multipoint (RS-485) systems require that the main cable
be terminated in its characteristic impedance at both ends.
Again, keep stubs connecting a transceiver to the main
SPEED OPTION
TABLE 4. RECOMMENDED STUB LENGTHS
SLOW
FAST
MED
17
MAXIMUM STUB LENGTH
100-150 (30.5 - 46)
350-500 (107-152)
1-3 (0.3 - 0.9)
ft (m)
OD
.
ISL81334, ISL41334
cable as short as possible, and refer to Table 4. Avoid “star”,
and other configurations, where there are many “ends”
which would require more than the two allowed terminations
to prevent reflections.
High ESD
All pins on the ISLX1334 include ESD protection structures
rated at ±4kV (HBM), which is good enough to survive ESD
events commonly seen during manufacturing. But the bus
pins (Tx outputs and Rx inputs) are particularly vulnerable to
ESD events because they connect to an exposed port on the
exterior of the finished product. Simply touching the port
pins, or connecting a cable, can destroy an unprotected port.
ISLX1334 bus pins are fitted with advanced structures that
deliver ESD protection in excess of ±15kV (HBM), without
interfering with any signal in the RS-485 or the RS-232
range. This high level of protection may eliminate the need
for board level protection, or at the very least will increase
the robustness of any board level scheme.
Small Packages
Many competing dual protocol ICs are available only in
monstrously large 24 to 28 Ld SOIC packages. The
ISL81334’s 28 Ld SSOP is 50% smaller than even a 24 Ld
SOIC, and the ISL41334’s tiny 6x6mm QFN is 80% smaller
than a 28 Ld SOIC.
Flow Through Pinouts
Even the ISLX1334 pinouts are features, in that the “flow-
through” design simplifies board layout. Having the bus pins
all on one side of the package for easy routing to a cable
connector, and the Rx outputs and Tx inputs on the other
side for easy connection to a UART, avoids costly and
problematic crossovers. Figure 11 illustrates the flow-
through nature of the pinout.
Low Power Shutdown (SHDN) Mode
The ON/OFF pin is driven low to place the IC (both ports) in
the SHDN mode, and the already low supply current drops to
as low as 25µA. If this functionality isn’t desired, the pin can
be left disconnected (thanks to the internal pull-up), or it
should be connected to V
1kΩ resistor. SHDN disables the Tx and Rx outputs, and
FIGURE 11. ILLUSTRATION OF FLOW THROUGH PINOUT
A1
B1
B2
A2
Y1
Y2
Z1
Z2
ISL81334
R
CC
D
(V
L
for the QFN), through a
RA1
DY1
DY2
RA2
µCONTROLLER
UART
ASIC
OR
OR
December 20, 2005
FN6202.1