E-Zine May 2006
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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
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
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