Data Acquisition

6The standardization of the RS-232 serial port as part of the IBM PC and its compatibles hasled to this communications interface being used for many stand-alone loggers and otherinstruments that have interfaced to the PC. With the advent of smart instrumentation such asdigital transmitters and their use in a distributed data acquisition and control system, the requirementsof interfacing multiple devices on a multi-drop network has led to the extensiveuse of the RS-485 communications interface.This chapter reviews the fundamental definitions and basic principles of digital serial datacommunications. It details two of the most popular and common interface standards used indata acquisition and control systems (RS-232 and RS-485). In addition, the most commonindustrial protocols are examined, including methods for detecting errors in communication,an important consideration in noisy industrial environments. A section on troubleshootingand testing serial data communications circuits has been included for completeness.In the past, Ethernet has been usually thought of as an office networking system. Nowmany manufacturers are using Ethernet and industrial fieldbuses as communication systemsto interconnect data acquisition devices. This can take the form, among others, of connectingcomputers that are using plug-in data acquisition cards or data loggers that are networkedtogether. Fieldbuses such as Profibus and Foundation Fieldbus are being used to interconnectdevices that are doing data acquisition. There are currently several hybrid analog and digitalstandards available for communication between field devices and between field devices and amaster system. Only a fully-compatible digital communication standard will provide themaximum benefits to end users and one such standard, currently being proposed, is theFoundation FieldBusAll data communications systems have the following components:• The source of the data (e.g. a computer). Also required is circuitry that convertsthe signal into one that is compatible with the communications link, called atransmitter or line driver