RS-485 Datasheet | Maxim Integrated

Features INTERFACE CIRCUITS Dec 01, 2000 APPLICATION NOTE 367 Explanation of Maxim RS-485 Features 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 ±15k V, 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 bees 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...