wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

35-6 Industrial Communication Systems
The majority of today’s FBPS have galvanic isolation between the fieldbus communication module
to the host and the field segment where the instruments are installed as this prevents disturbances and
noises from the field or host system and the bulk power supply, propagating from one system to another;
thus, minimizing the risk of damage to the host I/O card.
FBPS are designed for a range of conditions, and as a result, the power conditioner may have
voltage output between 12 and 28.V and 80–500.mA. The predominant factor affecting the available
power range and associated voltage/current available is the area classification into which it is sourced
if you wish to have live working on the full system, partial system (spurs only), or use gas testing and
similar restrictions at all times when doing maintenance.
35.8 Installation of Segment in Safe Areas
The fieldbus specifications state that a maximum of 32 devices can be installed on a single H1 network;
however, doing so would result in a very long macrocycle and higher risk than most end users would be
willing to accept should something happen to communications on that port. Consequently, and in part
because many host systems reach a parameter/port limit at this level, most designs use a maximum of
16 loop powered devices per segment.
Many projects therefore start their design basis with the maximum number of devices per network
of 12 devices per segment. Many designs have a corollary design rule that of these 12 devices, no more
than four control loops (valves) should be used.
Because fieldbus is wired in parallel, it is strongly recommended that the spurs have short-circuit
protection for all spur connections. This is so that a short circuit on a spur or in an instrument will
not cause a shutdown of the entire segment. Fortunately, almost all field device couplers have this
capability.
Many projects also develop a number of “standard” fieldbus enclosure configurations with a fixed
number of spurs and home run cables per junction box. This standardization makes the project design
easier, simplifies the installation, and optimizes the number of items that need to be carried in stock.
One other benefit is that many fieldbus device coupler manufacturers now also offer a standard range
of “preconfigured” junction boxes, so that all the project needs to do is mount the enclosure, which has
been specified and designed for the environment where it is to be installed and run the cables to the
appropriate terminations. The drawing here (Figure 35.3) shows the minimum size junction box specified,
which is a four-spur configuration.
Monitoring PC or DCS
H1
Junction box
Single twisted pair
FIGURE 35.3
Fieldbus foundation installation in the field.
© 2011 by Taylor and Francis Group, LLC