The type of communication protocol that is suitable for a SIL 2 or SIL 3 system is really dependent on the type of platform that is being used. Options include, but are not limited to: 4-20 mA output signal, ControlNet (Allen Bradley), DeviceNet Safety (Allen Bradley), SafetyNet (MTL), and PROFIsafe. Currently, the ISA SP84 committee is working on developing guidelines for a safety bus, to make sure that the foundations comply with IEC 61508, and IEC 61511 standards.
The first devices with a safety bus should be available by 2008. The Fieldbus Foundation is actively involved in the committee and working on establishing Foundation Fieldbus Safety Instrumented Systems (FFSIS) project to work with vendors and end users to develop safety bus specifications.