Lastly, the relative cost of the IS buses is high compared with NeSSI devices. The IS buses were designed to be deployed in larger process control valves and transmitters in which the bus technology represents a fraction of the device’s total cost. NeSSI devices typically cost much less than larger process control valves or transmitters, making the cost of a smart NeSSI device that uses one of the IS bus technologies much higher than its non-networked counterpart.
Other industries, such as semiconductor, consumer electronics and automotive, use simpler, smaller and less expensive buses such as USB, I2C and CAN, which would be ideal for use as the NeSSI bus; however, these options aren’t intrinsically safe.
Given these choices, decision-makers were faced with a conundrum: Do they make one of the IS buses simpler, smaller and less expensive, or do they make one of the simpler, smaller and less expensive buses intrinsically safe?
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:
- The link layer, usually implemented in protocol controller ASIC, defines the control bytes and their use in transporting the data;
- The application layer, usually implemented in software, defines the meaning of the data bytes
- 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.
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.