1. IS CAN Bus – CiA WD 103 Update
Low Cost, Intrinsically Safe Transducer Bus Enabling NeSSI Gen II
Rick Ales Swagelok Company
© 2006 Swagelok Company.

2. MPC Gen II Architecture
Ethernet
Remote Operator Station
Analyzer
Hazardous Area
Class 1 Div 1 (Zone 1)
THE NeSSI BUS CONUNDRUM
On the surface, choosing an open standard IS transducer bus seems to be a simple choice of one of four possible protocols: FOUNDATION fieldbus™, PROFIBUS PA, HART®, or ControlNet™. However, the NeSSI consortium discovered that none of these buses was particularly well suited for the types of smart devices that would populate a NeSSI Gen II system. The consortium had concerns about the architecture, size and cost of these options.
Architecturally, the IS buses do not match the simple design of NeSSI devices. Typically, the IS buses are designed to operate a few sophisticated devices in a complex messaging format over long distances (i.e., 1.9 km for FOUNDATION fieldbus). However, NeSSI Gen II components are simple sensors and actuators that support basic configuration and diagnostic functions, thereby requiring a less complex communications format. Further, the NeSSI devices communicate with a local controller typically located less than 10 meters away.
On the size front, the 1½-inch square ANSI/ISA footprint places a serious constraint on the amount of electronics that can be fitted onboard a NeSSI Gen II device. The major IS buses, with the exception of HART, require at a minimum: a relatively powerful microcontroller, a protocol controller ASIC (Application Specific Integrated Circuit), a bus interface, and driver circuits. The size and number of required electrical components makes it difficult, at best, to squeeze an IS bus – along with the requisite input/output (I/O) circuitry, connectors and other miscellaneous electronics – into the NeSSI footprint.
Lastly, the relative cost of IS buses compared to NeSSI devices is high. 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.
Industries not using process analytics, such as semiconductor, consumer electronics and automotive, use simpler, smaller and less expensive buses like USB and CAN. However, these options are not intrinsically safe.
Given the choices, decision makers are faced with the NeSSI bus 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?
IS NeSSI™ Bus V
Flow
Sensors P T A
NeSSI Device Sample System
Sensor Analyzer Manager SAM
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

3. Communications Model Virtual Message Path Actual Message Path
The Application layer represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.
The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one node to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer
The Physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

4. IS and the Ignition Curve
Intrinsic Safety
Intrinsically safe equipment is defined as "equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration." (ISA-RP12.6) This is achieved by limiting the amount of power available to the electrical equipment in the hazardous area to a level below that which will ignite the gases.
In order to have a fire or explosion, fuel, oxygen and a source of ignition must be present. An intrinsically safe system assumes the fuel and oxygen is present in the atmosphere, but the system is designed so the electrical energy or thermal energy of a particular instrument loop can never be great enough to cause ignition.
Ignition Curve
Group A & B = Acetylene + Hydrogen; C= Ethylene; D=Gasoline, Propane
IS bussed require 24VDC to achieve kilometer length distances; from ignition cure 24VDC corresponds to 200 ma and accounting for safety factors results in around 2.5 Watts of available power.
Relax the distance requirement to meter length distances then the required voltage can be reduced to 10 volts or less which allows current is higher power. A commercial power supply providing up to 4 channels at 9.5 VDC at 1 amp per channel is readily available.
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

5. IS CAN Major Parameters
The CAN signaling spec. ISO ,2 & CiA301
Co-cabled 10m max Single power source only
3.3V Transceiver 1/3 the Power
Power bus specification P+F RSD2-PSD-EX4.43
M12 or M8 Connectors 2 Power and 2 CAN wires
Area Classification Class 1 Div 1 (Zone 1)
Entity Parameters
VOC (V)
ISC (A)
Groups
CO (uF)
LO (uH)
A, B (IIC) (Acetylene & Hydrogen)
0.8 10
9.5Vdc
1.0 A
C (IIB) (ethylene)
2.4 40
D (IIA) (propane)
6.4 80
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

6. IS CAN Physical Layer Topology
4 Circuits
8.5 Watts each
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

7. CiA WD103 Organization Joshua Pearl IXXAT Co. Chairman
Reiner Zitzmann CAN in Automation Secretary
Swagelok
ABB
Siemens
Texas Instruments
Pepperl + Fuchs
Turck
Parker Hannifin
Circor
Prototype IS CAN system is running at ABB Lewisburg using the following components:
P+F Power supply
TI 3.3V transceiver and 2407 DSP evaluation systems
IXXAT isolated repeater
Current Draft standards has been submitted to CiA for distribution for comment.
© 2006 Swagelok Company. Swagelok confidential. For internal use only.

8. Discussion?
© 2006 Swagelok Company. Swagelok confidential. For internal use only.