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

Foundation Fieldbus 35-3
Four device classes are specified for HSE networks: (1) host devices (HD) are PCs or controller devices with
an Ethernet port, (2) a link device (LD) allows an connection from an Ethernet network to multiple H1
segments, (3) foreign I/O gateways are components for integrating third-party fieldbuses, and (4) Ethernet
devices (ED) that allow a direct integration to Ethernet networks.
35.4 Drivers (DD, EDDL, and FDT/DTM)
The majority of today’s “smart” analog instruments use EDDL (electronic device description language),
often shortened to DD (device description) in conversation. EDDL is the base technology underlying
the HART, Profibus PA, and FF protocols by defining a significant part of their user layer. DD files for
all approved FF devices are accessible from the Fieldbus Foundation Web site. Each device has a unique
set of three or four files in the zip file associated with the device. All of these files except the CFF, or
capabilities file, which is in plain ASCII are binary files.
The original EDDL specification as defined by IEC 61804 Part 2 contains little support for graphics;
thus, the interface and degree of integration between field devices and hosts was not only limited, but
the level of integration and information that could be presented to a user varied between each installation
depending on the host being used.
One technology that was developed to attempt to overcome these limitations was FDT/DTM (field
device tool/device-type manager). The FDT/DTM technology uses EDDL as the basis for its definitions,
so that device manufacturers can define the “look and feel” of how the user accesses the device information
for other than process variable (PV) and related signals, such as those used to configure and
calibrate the device.
35.5 Cables
The Fieldbus Foundation uses a pair of wires to supply the DC voltage and the communication signal of
the protocol. This cable is normally shielded to minimize noise on the segment. As shown in Table 35.1, the
fieldbus standard defines four possible cable types. The most commonly used cable is type “A.” When type
“A” cable is used, networks, or segments as they are commonly called, with total combined length (sum of
the trunk plus all spurs) of up to 1900.m are possible without the use of repeaters. The specification does,
however, allow for up to four repeaters to be used on a single segment.
If the designer chooses another cable type than type “A,” the available segment length will be reduced.
For example, if a type “B” cable (twisted pair with overall shield) were to be used, the total cable budget
available is reduced to 1200.m. Experience has shown that Ohm’s law is often the determining factor in
the limitation on overall length, so an effective way to be able to increase the length of a network to these
maximums is to use a larger diameter cable with less resistance.
The IEC 61158-2 specifies in Section 11.7.2 that “The cable used for testing Fieldbus devices with
a 31.25.kbps voltage-mode MAU for conformance to the requirements of this part of IEC 61158 shall
be a single twisted-pair cable with overall shield meeting the following minimum requirements at
25 deg C… subsection d) maximum d.c. resistance (per conductor) = 24 Ohm/km.” Then later in the
TABLE 35.1
Foundation Fieldbus Cables
Type Description Size Maximum Length
A Shielded, twisted-pair #18 AWG (0.8.mm 2 ) 1900.m (6232.ft.)
B Multiple, twisted-pair with shield #22 AWG (0.32.mm 2 ) 1200.m (3963.ft.)
C Multiple, twisted-pair without shield #26 AWG (0.13.mm 2 ) 400.m (1312.ft.)
D
Multi-core, w/o twisted pairs
and having overall shield
#16 AWG (0.1.25.mm 2 ) 200.m (656.ft.)
© 2011 by Taylor and Francis Group, LLC