4 - EIA-485 overview

Abstract

This chapter focuses on Electronic Industries Alliance (EIA)-485 standard and its features. The EIA-485A standard is one of the most versatile of the EIA interface standards. It is an extension of EIA-422 and allows the same distance and data speed but increases the number of transmitters and receivers permitted on the line. EIA-485 permits a “multidrop” network connection on two wires and allows reliable serial data communication for distances of up to 1200 m, data rates of up to 10 Mbps, up to 32 line drivers on the same line, and up to 32 line receivers on the same line. According to the EIA-485 standard, there can be 32 “standard” transceivers on the network. Some manufacturers supply devices that are equivalent to ½ or ¼ standard device, in which case this number can be increased to 64 or 128. If more transceivers are required, repeaters have to be used to extend the network. The EIA-485 interface standard is very useful for systems where several instruments or controllers may be connected on the same line.