DS485 Datasheet, PDF (7/10 Pages) National Semiconductor (TI) – Low Power RS-485/RS-422 Multipoint Transceiver

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.
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.
Multipoint RS-485 Application
DS012880-16
7
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