Intrinsically Safe NeSSI Nears

An emerging bus standard promises to spur application in hazardous environments.

By Rick Ales, Swagelok Co.

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A solution emerges
The NeSSI bus market is relatively small compared to others. So, it would be unrealistic to imagine that any one company would allocate resources to develop an IS transducer bus specifically for NeSSI. From the outset, the NeSSI visionaries realized they would need to build on existing bus systems and reuse as much commercially available technology and infrastructure as possible to have a realistic chance of success.

A key requirement of a NeSSI bus is that it be an open standard, available to the general public (not necessarily free of charge). To ensure compatibility, interoperability and multi-vendor component interchangeability, a bus must adhere to the three layers included in the Open Systems Interconnection (OSI) Layer Reference Model (ISO 7498). Typical industrial communications protocols implement the following three OSI layers:
1. The link layer, usually implemented in protocol controller ASIC, defines the control bytes and their use in transporting the data;
2. The application layer, usually implemented in software, defines the meaning of the data bytes; and
3. The physical layer defines the electrical signals, their timing and their transport media (i.e., wiring, connectors, etc.).

The link layer posed two principal concerns — cost and size (or miniaturization) of the ASIC. The cost of integrated circuits is proportional to their quantity and complexity. CAN, originally developed by Bosch for the automotive industry, is a link layer specification implemented in ASICs and microcontrollers produced in huge quantities — more than 500 million CAN chips are sold each year. With high production volumes and simple architecture, CAN network controllers often are included on simple 8-bit microcontrollers costing less than $5. Further, with the communications controller embedded in the same IC as the microcontroller, the miniaturization achievable with the CAN protocol is unparalleled by any network requiring separate protocol and processing ICs. These characteristics make CAN a good choice for the NeSSI bus link layer.

Figure 3
Function Block Model: CANopen provides device profiles and function block descriptions.
Source: CiA.

The principal challenge with the application layer was to ensure interoperability and enable interchangeability in an open multi-vendor environment. CAN in Automation (CiA) members have developed a number of device profiles that specify how transducers and closed-loop controllers connect, enabling a plug-and-play network. CANopen device profiles such as DS 404 V1.2 describe devices for measuring or controlling loops of different physical quantities. Also defined are function block descriptions of digital input, analog input, digital output, analog output, controller, alarm and device functions (Figure 3). Availability of CiA device profiles made CANopen a good choice for the NeSSI bus application layer.

Given the suitability of CANopen link and application layers, intrinsic safety remained the only issue to be resolved. The suitability of a system for IS applications is determined largely by its external electrical signals, which in a communication system are defined in the physical layer specification.

Figure 4
Safe Oerating Current:Bus operating at 24V dc significantly limits current available to IS device.

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).

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