Intrinsically Safe NeSSI Nears

An emerging bus standard promises to spur application in hazardous environments

By Rick Ales, Swagelok Co.

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In the physical layer, the principal challenge was to define bus operation at a low enough voltage and current to ensure the bus would be intrinsically safe. Field buses are designed to work over long distances; therefore, their bus voltage must be high (nominally 14–24 V dc) to accommodate the signal attenuation due to the high impedance associated with long wire runs. However, long wire runs aren’t required in NeSSI systems, so the line impedance is lower, decreasing the potential signal attenuation.

This means the NeSSI bus can operate with much lower voltage than field buses. In a NeSSI system, networked devices are clustered in a small area, usually less than 3 ft × 3 ft, and typically are closer than 10 meters to a safe area. The NeSSI consortium realized that this short physical network length meant bus voltage could be reduced significantly from the standard 24 V dc required to drive signals up to a mile or more. The short distance of the NeSSI bus permits a more manageable 10 V dc voltage, allowing a fivefold increase in the power budget while maintaining IS feasibility (Figure 4).

New BiCMOS technology enables low-power CAN electronic systems that use about one-third the power of ISO 11898 CAN driver circuits. Further, the BiCMOS CAN driver components remain compatible with existing CAN systems. Thus a NeSSI bus solution seemed within reach. By placing restrictions on the wiring distance and using BiCMOS drivers, CAN would be made suitable for IS applications.

Moving ahead
However, further progress had to wait until the NeSSI consortium could get a standards organization to take ownership of the proposed IS CAN physical layer development and specification. Finally, in 2003, a NeSSI workshop, hosted by the National Institute of Standards and Technology, uncovered the opportunity to address the issue of ownership. A new project — known as IEEE P1451.6 — had just been commissioned to harmonize the IEEE 1451 family of smart transducer interface standard Transducer Electronic Datasheets (TEDS) with the CANopen standard device profile DS 404. As a result of this workshop, an option to define a low-power physical layer interface to address the NeSSI IS requirement was included in the IEEE P1451.6 project.

In March 2006, the NeSSI consortium transitioned the IS CAN project into the CiA and composed Working Draft 103, “CANopen Intrinsically Safe Physical Layer,” which, after comment and refinement, was released as Draft Standard “CiA 103 DSP V1.0: CANopen intrinsically safe capable – Physical layer specification.” It’s available on the CiA Web site (www.can-cia.org/downloads/ciaspecifications/).

CANopen, with the 103 DSP specification, has become the NeSSI bus solution envisioned nearly six years ago. Today, CANopen IS transducer bus NeSSI systems are in the process of development and evaluation in a number of pilot projects (Figure 5).

Broad promise
The NeSSI consortium has shown that a low-cost transducer bus can be intrinsically safe. CiA device profiles and CANopen conformance testing furnish the infrastructure by which transducer vendors can offer certified plug-and-play multi-channel devices for cost-effective sensor and actuator solutions. The CiA also provides a forum for special interest groups, such as NeSSI, to adjust the CANopen protocol to meet their specific industry requirements. Emerging standards such as CiA 103 DSP V1.0 make a strong case for end-user adoption of a CAN protocol in hazardous process environments.

The process industries won’t be the only beneficiary of the CiA IS CAN standard. It’s a technically viable solution for many other applications. Opportunities for IS systems can exist where CAN-open communications are already deployed, such as for paint booth robots in automobile manufacturing. If CiA IS CAN is added to the ISO 11898 family of standards, it will enable other CAN-based automation protocols with intrinsic safety. The future for IS CAN is promising.



Rick Ales is product manager at Swagelok Co., Solon, Ohio. E-mail him at Richard.Ales@Swagelok.com.

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