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E-Zine May 2006
Click here to review Part 1 The basic network for FOUNDATION™ Fieldbus is H1, a rather low speed serial protocol designed to deliver power to process control field devices somewhat in excess of similar analog or HART instruments operating using 4-20mA DC. Power is inexorably defined with considerations of intrinsic safety. When intrinsic safety is required, the number of devices that can be powered from a fieldbus segment is reduced. Process control instruments, which include transmitters that sense physical, electrical, and chemical properties, and final control elements, such as control valve positioners and variable speed drives, are interconnected by H1. A single H1 bus segment may address up to 255 field devices, but it is typically limited to far fewer due to the demands to supply power, intrinsic safety considerations, and the network data transmission limitations of the H1 speed. The original design of the Fieldbus standard assumed "worst case" conditions for the purposes of specifying power to the field devices, that is, that they would always demand maximum power and were installed in Division 1 (North American rating) or Zone 1 (IEC rating) hazardous locations. This is called the Entity Concept for bus power design. FOUNDATION™ Fieldbus H1 can, but need not be intrinsically safe. The Fieldbus Intrinsically Safe Concept (FISCO) allows H1 segments to be intrinsically safe with up to 215 milliamps at a nominal 15 volts DC. An intrinsic safety barrier is required at the boundary between the hazardous area and the safe area. FISCO allows more intrinsically safe field devices to be powered from the H1 bus than with the Entity Concept. The FISCO solution is based on experimental work that showed that the inductive effects of the Fieldbus cable could be ignored in computing the maximum current capacity for an intrinsically safe H1 segment. The Fieldbus Non-Incendive Concept (FNICO) is based on the realization that most process areas are rated Division 2 or Zone 2, so the hazard is present only under abnormal conditions. This allows higher currents to be used, although the need for barriers is still present. As a result, more FOUNDATION™ Fieldbus H1 or PROFIBUS-PA instruments can be connected per segment than using the Entity Concept or FISCO. H1 specifications utilize a noise-resistant modulation called Manchester Bi-phase encoding. Because each symbol (a one or a zero) requires two changes of state, it is tolerant of randomly generated noise. Additionally, each pulse has a trapezoidal waveform to enable high frequency random noise and impulses to be filtered from the detection circuits. In actual network designs each Fieldbus H1 segment typically contains the instruments necessary for a single control loop plus (perhaps) a few unrelated measurement devices. This is done to maintain the single loop integrity. For example, if an H1 segment fails, no more than a single loop of control would be lost. Since the days of pneumatic controls, this has been an acceptable philosophy to control processes. Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control ISSN 1538-5280 |
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