RS-232

Abstract

RS-232 is one of the most widely used techniques used to interface external equipment to computers. It uses serial communications where one bit is sent along a line, at a time. This differs from parallel communications which send one or more bytes, at a time. The main advantage of serial communications over parallel communications is that a single wire is need to transmit and another to receive. RS-232 is a de facto standard that most computer and instrumentation companies comply with. The Electronics Industries Association (EIA) standardized it in 1962. Unfortunately, this standard only allows short cable runs with low bit rates, such as a bit rate of 19 600 bps for a maximum distance of 20 meters. New serial communications standards, such as RS-422 and RS-449, allow very long cable runs and high bit rates. For example, RS-422 allows a bit rate of up to 10 Mbps over distances up to one mile, using twisted-pair, coaxial cable or optical fibers. The new standards can also be used to create computer networks. This chapter introduces the RS-232 standard and gives simple programs which can be used to transmit and receive using RS-232. Chapter 48 uses an interrupt-driven technique to further enhance the transmission and reception.