DS485_15 Datasheet, PDF (8/16 Pages) Texas Instruments – Low Power RS-485/RS-422 Multipoint Transceiver

DS485
SNLS122C – JULY 1998 – REVISED APRIL 2013
APPLICATIONS INFORMATION
www.ti.com
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 Related TI Low Power RS-485 Transceivers). The DS485 also ensures the driver's output
differential voltage 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 receivers.
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 temperature 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 figure. Note that termination is typically required but is
only located at the two ends of the cable (not on every node). Commonly pull up and pull down resistors may be
required at one end of the bus to provide a failsafe bias. These resistors provide a bias to the line when all
drivers are in TRI-STATE. See Application Note AN-847(SNLA031) for a complete discussion of failsafe biasing
of differention buses.
Figure 15. Multipoint RS-485 Application
8
Submit Documentation Feedback
Product Folder Links: DS485
Copyright © 1998–2013, Texas Instruments Incorporated