March 2006

Special Report: Instruments and Networks

Not all safety integrity level 3 safety systems are the same

Most of the differences relate to online availability

Gruhn, P., ICS Triplex, a Rockwell Automation Company

Many technologies have been used in safety system applications. First there were relays: a fail-safe, simple and proven technology. Solid-state systems, introduced in the 1960s, were designed to replace relays. These systems offered additional features yet didn't utilize software. Programmable logic controllers (PLCs) were introduced in 1969. These systems were also designed specifically to replace relays yet did utilize software. Some recognized the inherent problems with general-purpose PLCs in safety applications, primarily due to their lack of effective diagnostics. Specialized triplicated PLCs for critical control and safety were released in the early and mid-1980s. Competition among

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