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 sends one or more bytes, at a time. The main advantage that serial communications has over parallel communications is that a single wire is needed to transmit and another to receive. RS-232 is a de facto standard that most computer and instrumentation companies comply with. It was standardized in 1962 by the Electronics Industries Association (EIA). Unfortunately this standard only allows short cable runs with low bit rates. The standard RS-232 only allows a bit rate of 19 600 bps for a maximum distance of 20 metres. 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 1 mile, using twisted-pair, coaxial cable or optical fibres. 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. The following chapter shows how Turbo Pascal can be used to transmit data through the parallel port.