Current loop and RS-485 converters overview

This chapter provides an insight into current loop hardware and RS-485 interface converters. Current loop is a commonly used interface technique that uses a current signal rather than a voltage signal, employing a separate pair of wires for the transmitter current loop and receiver current loop. Current loops are mainly used between printers and terminals in the industrial environment. Serial interface converters are becoming increasingly important with the move away from RS-232C to industrial standards such as RS-422 and RS-485. Because many industrial devices still use RS-232 ports, it is necessary to use converters to interface a device to other physical interface standards. Interface converters can also be used to increase the effective distance between two RS-232 devices. The RS-232/422 and RS-232/485 interface converters are very similar and provide bidirectional full-duplex conversion for synchronous or asynchronous transfer between RS-232 and RS-485 ports. These converters may be powered from an external AC source, such as 240 V, or smaller units can be powered at 12 V DC from pins 9 and 10 of the RS-232 port.