Introduction to FIELDBUS

An Introduction to FIELDBUS.

As a starting point we are talking about FOUNDATION FIELDBUS H1 (31.25 kbit/sec), which is comparatively easy to understand. FIELDBUS network contains twisted pair wire, carrying both a digital signal and DC power that connects up to 32 fieldbus devices - theoretically  (temperature, flow, level and pressure transmitters, smart valves, actuators, etc.) to a DCS or similar control system. Most devices are two-wire bus-powered units requiring 10 to 20mA, but it is also possible to have 4-wire fieldbus devices, typically where a device has a particularly high current draw.

Basic fieldbus loop contains:
       1. H1 interface card- communication link between segment and host system.
       2. Power supply-Powering & regulates DC voltage on loop.
       3. Twisted cable network-Prevents cross-talk
                             Max. 1900 mtr/Segment, Max bus voltage 32V.
       4. Junction box (optional)- to accommodate devices effectively.
       5. Field instruments -collects process variables
                            Wire length Max. 120MTR From segment.
                            Operating voltage Min. 9V,
       6. Terminator-prevents signal reflections.

The DC power required by the bus is normally sourced through a fieldbus power supply or “power conditioner” which prevents the high frequency communications signal from being shorted out by the DC voltage regulators. Typical power conditioners make 350 to 500mA available on the bus and usually incorporate isolation to prevent segment-to-segment cross talk. The power conditioners are separate from the H1 interface card and are often installed in redundant pairs to improve the overall reliability. Figure shows a typical fieldbus segment.

Number of factors limits no. field devices:
 Current requirement of each devices.
 Length of segment- 9V must reach farthest end of segment.
 Power conditioner use.
 Type of cable use- Voltage drop in the cable.
Safety measures in Fieldbus.
In all industrial setups there will be a safety margin that may be overlapped on theoretically safe region. In fact number of devices in segment further decrease. During network designing time engineers consider "fail safe" nature of system. Network failure makes decrease the current availability. So that max. 16 devices are connected in the segment.      
Fieldbus has one major problem: all communications and power are dependent upon a single twisted pair trunk cable. If the trunk cable fails, it can “take down” all the devices on the segment at one time. Not only is the fieldbus segment lost to the control system, devices on
the segment can no longer talk to each other. Although fieldbus instruments can continue to operate if the control system fails, any cable fault (open or short-circuit) could render the entire segment inoperable.
Various fieldbus vendors, including major process control companies, have developed redundant fieldbus schemes that involve complete duplication of all equipment—including H1 or DP/PA interfaces, power supplies, fieldbus cables, device couplers, and some critical fieldbus instruments —plus complex software voting schemes. Such redundancy schemes are expensive, complex, and can be hard to maintain. A catastrophic process failure could result while the control system is determining what’s wrong.

FIELDBUS Having following advantages.
•Wiring— They achieved a 98% reduction in home-run wiring because with fieldbus they could eliminate the costly maze of wiring between each remote field instrument and the control room. Terminations were also reduced by 84%.
• Control room—Fewer terminations also freed up two-thirds of the cabinet space that would be required with traditional technology.
• Commissioning— Field check-out and QA/AC time was reduced by 83%. Installation of each
transmitter took only 20 minutes rather than the two hours needed with non-fieldbus technology.
• Engineering drawings— Reduced the effort required for new drawings when adding oil wells by 92% because of FOUNDATION fieldbus and the host system’s configuration tools and object-oriented capabilities.

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