herunterladen
Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 367
Keywords: RS-485,rs485, transceivers, EIA/TIA-485, RS-422, rs422, EIA/TIA-422, 12Mbps, 2.5V supply,
unit-load, ESD protection, protected, +/-15kV ESD, half-duplex, full-duplex, cable termination, fail-safe,
low-power shutdown
APPLICATION NOTE 367
Explanation of Maxim RS-485 Features
Dec 01, 2000
Abstract: Features are described for Maxim's large selection of RS-485 transceivers. Devices are
identified to meet requirements for half- or full-duplex operation, supply voltages down to 2.5V, ESD
protection up to ±15kV, data rates from 250kbps to 12Mbps, slew-rate limited devices for low radiated
noise operation, fail-safe operation with open or shorted cables, normal 1 unit-load to 1/8 unit-load for
expanded networks, and low-power shutdown operation.
Maxim makes what seems like a bewildering array of RS-485 drivers and receivers. This application
note explains the different features within Maxim's product line and thus will help you in selecting the
most appropriate part for your needs. Below are explanations of the various features and other important
things to keep in mind.
Data Rates and Slew-Rate Limiting
At some time during the design of an RS-485 system, a maximum data rate appropriate for that system
is determined. Then it becomes necessary to choose the physical RS-485 devices that are capable of
running at this speed. Devices are selected that are rated for speeds equal to or greater than the
required data rate. Knowing this, you might wonder if there are any disadvantages in choosing the fastest
devices available, if they might be overkill. The answer is yes!
Although it is true that fast devices can be used for both high data rates and low data rates, there are
drawbacks in using devices that are faster than needed, including higher radiated emissions and greater
susceptibility to improper terminations.
Radiated emissions occur because the wiring used in RS-485 will act as an antenna. RS-485 tries to
minimize radiated emissions by using twisted-pair cabling and balanced transmitters. The idea behind
this is that the balanced transmitter, as the name implies, will generate two equal but opposite signals
that are sent down the two wires in a twisted pair. Because the wires are virtually on top of each other,
they will tend to radiate the exact opposite signal that the other wire is transmitting. This has the effect of
canceling each other out, and ideally results in no net radiated emissions. This tends to work fairly well,
but, like everything in engineering, it isn't perfect. Inevitably, some radiated emissions will leak out. As a
general rule, the higher the frequency components in the signal and the longer the cable, the worse the
situation becomes.
The same applies to cable terminations. Because of the distances of the cabling and the data rates,
careful attention must be paid to transmission-line effects. Both ends of an RS-485 cable should be
properly terminated in the characteristic impedance of the cable to prevent reflections. Resistor and cable
Page 1 of 8
