AN736| Application Note
Maxim > App Notes > INTERFACE CIRCUITS Keywords: RS-485, RS485, RS-422, RS422, EIA/TIA-485, EIA/TIA-422, differential data transmission, fullduplex, half-duplex, unit load, fractional unit load, EMI, ESD protection, electro-static discharge, electromagnetic interference
Mar 16, 2001
APPLICATION NOTE 736
RS-485 (EIA/TIA-485) Differential Data Transmission System Basics
Abstract: Electrical specifications are presented for the RS-422 unidirectional, RS-485 full-duplex and PROFIBUS data transmission systems. Electrostatic discharge (ESD) protection, data rates, and loading conditions are considered. The concept of fractional unit loading is introduced. The RS-485 standard was developed jointly by two trade associations: the Electronic Industries Association (EIA) and the Telecommunications Industry Association (TIA). The EIA once labeled all its standards with the prefix "RS" (Recommended Standard). Many engineers continue to use this designation, but the EIA/TIA has officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards. Today, various extensions of the RS-485 standard accommodate a large variety of applications. The RS-485 and RS-422 standards share many of the same attributes, and are often confused for that reason. The table below compares the different standards. The RS-485, which specifies bi-directional and half-duplex data transmission,
is the only EIA/TIA standard that allows multiple receivers and drivers in "bus" configurations. Conversely RS-422 EIA/TIA standard specifies a single, unidirectional driver with multiple receivers. Therefore RS-485 parts are backward compatible and interchangeable with their RS-422 counterparts. However, RS-422 drivers should not be used in an RS-485 system for they cannot relinquish control of the bus. RS-485 and RS-422 Standards RS-422 Mode of operation Allowed no. of Tx and Rx Maximum cable length Maximum data rate Minimum driver output range Maximum driver output range Tx load impedance Rx input sensitivity Maximum Rx input resistance Rx input voltage range Rx logic high Rx logic low RS-485 PROFIBUS
Differential Unidirectional Differential Full Duplex Differential Full Duplex Multipoint Multipoint Multipoint 1 Tx, 10 Rx 4000ft length 10Mbps 2V 5V 100!& 200mV 4k!& 7V >200mV <200mV 32 Tx, 32 Rx 4000ft length 10Mbps 1.5V 5V 250mA 54!& 200mV 12k!& -7V to +12V >200mV <200mV 32 Tx, 32 Rx 4000ft length 40Mbps 2.1V 5V 250mA 54!& 200mV 12k!& -7V to +12V >200mV <200mV
Maximum driver short-circuit current 150mA
PROFIBUS
PROFIBUS is a communication standard built upon an RS-485 type transceiver. It is intended for industrial control networks. The PROFIBUS specification covers the software protocol, the connector type, and a differential
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