ISA50, Signal Compatibility of Electrical Instruments

Scope

1. To develop standards for signals used in process measurement and control, to transmit information between subsystems or separated elements of systems. Such signals may be analog or digital. However, data highway and other time-shared transmission paths, such as those within the scope of ISA72, are excluded.

2. To continue to serve as the U.S. Technical Advisory Group for IEC-SC65A/WG4: Interface Characteristics.

Purpose

To define a common interface between components of electronic measurements and/or control systems.

A Word from the Chairman

Richard Caro, Chairman of ISA50 since 1993, and Cullen Langford, Chairman from 1987 to 1993, wish to thank everyone who participated in the development of the ISA-50.02 Fieldbus standards. The standards were administratively withdrawn as ANSI standards and have now also been withdrawn as ISA standards as they are no longer in use due to the issuance and acceptance of a more current IEC 61158 standards series.

ANSI/ISA-50.00.01-1975 (R2002), Compatibility of Analog Signals for Electronic Industrial Process Instruments is one of the most widely used standards in industrial automation (print and downloadable versions are available), specifying 4-20 mA as the signaling range for analog data transmission of instrumentation and control signals. The standard document is simple, and the technology is very mature. All ANSI standards must be either withdrawn or re-certified every five years. There have been no technical innovations in this area since 1972, and the trend is to move gradually to digital fieldbus transmission, however, the annual volume of instruments sold with an ISA-50.1 conforming analog signal is still much larger than all fieldbus instruments combined.

There have been some proprietary additions to the 4-20 mA implementations, particularly to use the band below 4 mA but above zero to signal a specific system malfunction, and likewise to use a small band above 20 mA as well. There has been no general call to standardize on these small additions, however. The standard is in the process of being reaffirmed.

Although the ISA50.4 subcommittee had been inactive for several years, ISA-TR50.02, Part 9, Fieldbus Standard for Use in Industrial Control Systems: User Layer Technical Report (downloadable version available), was published in 2000. The document is a valuable tutorial that captures the knowledge of a unique expert along with the analysis of a complete synthesized digital control system. Former ISA50 Chair Cullen Langford was integral in converting the text, almost 1000 pages, into a workable electronic format. The subcommittee and ISA50 thank him for his remarkable efforts.

See all ISA50 Standards

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