DS485N National Semiconductor, DS485N Datasheet - Page 7

• Manufacturer Part Number: DS485N
• Description: Transceiver IC
• Manufacturer: National Semiconductor

Specifications of DS485N

• No. Of Drivers: 1
• Driver Case Style: DIP
• No. Of Pins: 8
• Operating Temperature Range: 0°C To +70°C
• I/o Type: Driver/Receiver
• Transceiver Type: Multipoint
• Mounting Type: Through Hole
• No. Of Channels: 1
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Applications Information
The DS485 is a low power transceiver designed for use in
RS-485 multipoint applications. The DS485 can transmit
data up to 2.5 Mbps based on a ratio of driver transition time
to the unit interval (bit time) of 10%. This maximum data rate
may be further limited by the interconnecting media. The
DS485 provides a standard unit load to the RS-485 bus
across the common mode range of −7V to +12V. This allows
up to 32 transceivers (standard unit load) to be connected to
the bus. More transceivers may be connected to the bus if
they support a reduced unit load (see DS36C278). The
DS485 also guarantees the driver’s output differential volt-
age into a worst case load that models standard termination
loads and 32 unit loads referenced to the maximum common
mode voltage extremes. With a minimum of 1.5V swing into
this load, a 1.3V differential noise margin is supported along
with the standard common mode rejection range of the re-
ceivers.
Multipoint RS-485 Application
7
Due to the multipoint nature of the bus, contention between
drivers may occur. This will not cause damage to the drivers
since they feature short-circuit protection and also thermal
shutdown protection. Thermal shutdown senses die tem-
perature and puts the driver outputs into TRI-STATE if a fault
condition occurs that causes excessive power dissipation
which can elevate the junction temperature to +150˚C.
A typical multipoint application is shown in the following fig-
ure. Note that termination is typically required but is only lo-
cated at the two ends of the cable (not on every node). Com-
monly pull up and pull down resistors may be required at one
end of the bus to provide a failsafe bias. These resistors pro-
vide a bias to the line when all drivers are in TRI-STATE. See
National Application Note 847 for a complete discussion of
failsafe biasing of differention buses.
DS012880-16
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