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

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