DS16F95A Datasheet, PDF (7/10 Pages) National Semiconductor (TI) – TIA/EIA-485-A (RS-485) Extended Temperature Differential Bus Transceiver

Functional Description
The Differential Line Driver levels shifts standard TTL/CMOS
levels to a differential voltage on the bus pins (A and B) that
conform to RS-485. The driver is enabled when the DE pin is
High. The driver is disabled when the DE pin is Low. The DI
and DE pins should be driven or tied to the desired state, do
not float. The differential driver is able to source and sink up
to 60mA of output current. Care should be taken that the driver
is not enabled into a fault condition where the package power
dissipation capacity is exceeded. The DS16F95A features
driver current limiting (see I_OS specification) to protect from
certain line faults where the amount of power is limited. This
device is intended for use in rugged applications at elevated
temperatures. It does not include a Thermal Shutdown fea-
ture commonly found on RS-485 transceivers.
The Differential line Receiver levels shifts the RS-485 levels
to standard TTL/CMOS levels. The receiver is enabled when
the RE pin is Low. The receiver is disabled when the RE pin
is High. The RE pin should be driven or tied to the desired
state, do not float.
Typical Application
A typical application is shown below. The RS-485 network
may be a simple point-to-point connection with two nodes or
a more complex one with up to 32 single unit load transceivers
as shown above. Stub lengths off the main line should be kept
as short as possible to minimize reflections. The line is ter-
minated at both ends in its characteristic impedance (typically
100 or 120 Ohms). The RS-485 network is a bi-directional half
duplex interface.
Being a multipoint bus, it is possible for all drivers to be dis-
abled when one or more receivers are enabled. In this case,
the receiver(s) is enabled when a valid differential voltage is
not present and its output state is unknown. A common solu-
tion is to provide external failsafe biasing to bias the line to a
known state such that the enabled receives will detect it cor-
rectly and idle with a static known state in this condition. See
AN-847 for a discussion on Failsafe biasing of differential
buses.
For extended temperature applications, maximum junction
temperature should be calculated. T_Jmax = T_A + (ThetaJA)
(Power Dissipation). Theta JA is the reciprocal of the derate
term ( 1 / 8.7 mW/°C or 115 °C/W). Recommended maximum
junction temperature for short duration operation is 200°C.
See AN-336 for a discussion on thermal considerations.
For maximum performance, a few system / PCB recommen-
dations are: drive the logic inputs (DI, DE, RE) with rail-to-rail
levels. This will provide the maximum noise margins to the
thresholds. A clean supply is also desirable, a 0.1µF capacitor
is recommended to be placed near the V_CC pin along with a
bulk capacitor. The use of power and ground planes is also
recommended. Stub lengths off the RS-485 interface should
be minimized to limit reflections. Typical interconnect
impedance is 100 Ohms.
Connection Diagram
8-Lead Dual-In-Line Package
30066719
30066701
7
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