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

35-4 Industrial Communication Systems
specification, B.1 Cable description and specifications, “The preferred Fieldbus cable is specified in 11.7.2
for conformance testing, and it is referred to as type ‘A’ Fieldbus cable.” Therefore, it is possible to, as we
just described, use larger diameter cable and remain in compliance with the standards.
To reduce the possible confusion in this area, the Fieldbus Foundation has recently released specification
FF-844 “H1 Cable Test Specification.” When manufacturers submit their cable for testing against this
standard, they can obtain an FF “check mark” for compliance and this will provide the end user a high
level of confidence that when properly installed, this cable will result in reliable fieldbus communications.
The FF-841 standard supports the use of single and multi-pair cables for fieldbus H1 communications.
35.6 Segment Design
One of the unique features of the FF technology is that it allows “control in the field.” However, to do
so, just like was the case with pneumatic control, all the elements of the control loop must reside on the
same segment or cable. Because the analog input (AI) and analog output (AO) devices must be on
the same cable for “control in the field,” even if you are not planning to implement this feature from the
beginning of the project, the segments should be designed so that you can do so in the future without
needing to physically rearrange your network.
When starting the segment design component of a FF project, you should have on hand the following
minimum information:
• Plant layout—for the approximate position of the instruments and the routing of the cable trays
in 3D, to determine approximate cable lengths.
• Process and instrumentation diagram (P&ID)—for the assignment of appropriate field devices to
the same segment.
• Area classification drawing—because it has an impact on the type of power conditioners that can
be selected and used.
Other decisions that you will need to make prior to starting the design include
• Maximum number of points per segment
• Protection method to be used. Will it be intrinsic safety (IS) or its equivalent, explosion proof or
other means
• Number of spares for future expansion
• Basic network design (tree/chicken foot) or some other arrangement
Figure 35.1 shows the tree or chicken foot arrangement, which is the most widely used design basis
for fieldbus networks.
The configuration has a “trunk” defined as the wire pair between the two terminators, with spurs to
each of the individual field devices.
PC, PLC, or DCS
Single twisted pair
H1
Junction box
H1
FIGURE 35.1
Foundation fieldbus network topology.
© 2011 by Taylor and Francis Group, LLC